U.S. patent application number 12/063345 was filed with the patent office on 2010-10-21 for nanorod thin-film transitors.
Invention is credited to Henning Sirringhaus, Baoquan Sun.
Application Number | 20100264403 12/063345 |
Document ID | / |
Family ID | 34984366 |
Filed Date | 2010-10-21 |
United States Patent
Application |
20100264403 |
Kind Code |
A1 |
Sirringhaus; Henning ; et
al. |
October 21, 2010 |
NANOROD THIN-FILM TRANSITORS
Abstract
A method for forming an electronic switching device on a
substrate, wherein the method comprises depositing the active
semiconducting layer of the electronic switching device onto the
substrate from a liquid dispersion of ligand-modified colloidal
nanorods, and subsequently immersing the substrate into a growth
solution to increase the diameter and/or length of the nanorods on
the substrate, and wherein the as-deposited nanorods are aligned
such that their long-axis is aligned preferentially in the plane of
current flow in the electronic switching device.
Inventors: |
Sirringhaus; Henning; (Coton
Cambridge, GB) ; Sun; Baoquan; (Cambridge,
GB) |
Correspondence
Address: |
FISH & RICHARDSON PC
P.O. BOX 1022
MINNEAPOLIS
MN
55440-1022
US
|
Family ID: |
34984366 |
Appl. No.: |
12/063345 |
Filed: |
August 9, 2006 |
PCT Filed: |
August 9, 2006 |
PCT NO: |
PCT/GB06/02981 |
371 Date: |
June 9, 2010 |
Current U.S.
Class: |
257/24 ; 257/43;
257/E21.461; 257/E29.094; 257/E29.245; 438/104; 977/773;
977/938 |
Current CPC
Class: |
H01L 21/02524 20130101;
H01L 29/0673 20130101; H01L 29/66742 20130101; H01L 29/786
20130101; B82Y 10/00 20130101; H01L 21/02521 20130101; H01L
21/02554 20130101; H01L 21/02628 20130101; H01L 29/7869 20130101;
H01L 21/02603 20130101; H01L 29/78681 20130101; H01L 21/02565
20130101 |
Class at
Publication: |
257/24 ; 438/104;
257/43; 257/E29.245; 257/E21.461; 257/E29.094; 977/773;
977/938 |
International
Class: |
H01L 29/775 20060101
H01L029/775; H01L 21/36 20060101 H01L021/36; H01L 29/22 20060101
H01L029/22 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 9, 2005 |
GB |
0516401.7 |
Jul 21, 2006 |
GB |
0614553.6 |
Claims
1. An electronic switching device having a semiconducting layer
that comprises inorganic semiconducting elongate nanoparticles
having a longer dimension and a shorter dimension, the average
ratio of the length of the longer dimension to the length of the
shorter dimension for the nanoparticles of the layer being in the
range 2 to 50 and the average length of the longer dimension of the
nanoparticles of the layer being less than 1000 nm, wherein the
nanoparticles of the layer are generally mutually aligned.
2. The electronic switching device of claim 1, wherein the
electronic switching device further comprises first and second
contacts defining a current transport path through the
semiconducting layer extending therebetween, the nanoparticles
being generally aligned along the direction of the current
transport path.
3. The electronic switching device of claim 2, wherein the distance
between the first and second contacts defines a channel length and
the ratio of the channel length to the average length of the longer
dimension of the nanoparticles of the layer is larger than 1.
4. The electronic switching device of claim 1, wherein the
nanoparticles are uniaxially aligned within one or more domains
having a diameter greater than 1000 nm.
5. The electronic switching device of claim 1, wherein at least
some of the nanoparticles are fused together.
6. The electronic switching device of claim 1, wherein the
nanoparticles comprise an oxide semiconductor.
7. The electronic switching device of claim 6, wherein said oxide
semiconductor is zinc oxide, tin oxide, zinc tin oxide, indium
oxide, zinc indium oxide or indium gallium zinc oxide.
8. The electronic switching device of claim 1, wherein the mobility
of the semiconducting layer is at least 0.5
cm.sup.2V.sup.-1s.sup.-1.
9. The electronic switching device of claim 1, wherein the density
of the film is at least 50% of the bulk density of the inorganic
material of which the nanoparticles comprise.
10. A method for fabricating a film of nanoparticles on a
substrate, the method comprising: forming a dispersion of elongate
inorganic nanoparticles in a solvent, the nanoparticles having one
or more ligand molecules attached to their surface, the
nanoparticles having a longer dimension and a shorter dimension,
and the ligand molecules including a functional group that enhances
the stability of the dispersion of the nanoparticles in the
solvent; and causing the nanoparticles to be deposited onto the
substrate from the dispersion by removal of the solvent at a
surface of the dispersion.
11. The method of claim 10, wherein the ligands are organic or
partly organic.
12. The method of claim 11, wherein said semiconducting
nanoparticles comprise an oxide semiconductor.
13. The method of claim 12, wherein said oxide semiconductor is
zinc oxide, tin oxide, zinc tin oxide, indium oxide, zinc indium
oxide or indium gallium zinc oxide.
14. The method of claim 10, wherein the ligands are selected so as
to cause the concentration of nanoparticles to be higher at the
surface of the solution than in the bulk of the solution, and the
shape of the nanoparticles is selected so as to promote mutual
alignment of the nanoparticles.
15. The method of claim 10, wherein the ratio of the length of the
longer dimension to the length of the shorter dimension for the
nanoparticles in solution is in the range 2 to 50 and the average
length of the longer dimension of the nanoparticles in solution is
less than 1000 nm.
16. The method of claim 10, wherein the density of the film is at
least 50% of the bulk density of the inorganic material of which
the nanoparticles are comprised.
17. The method of claim 10, wherein the solvent is removed at a
surface of the solution in such a way so as to define a direction
of preferential orientation for the nanoparticles and cause the
nanoparticles to become at least partially aligned along that
direction.
18. The method of claim 17, wherein the film defines a geometric
plane and the direction is out of the geometric plane defined by
the film.
19. The method of claim 17, wherein the film defines a geometric
plane and the direction lies in the geometric plane defined by the
film.
20. The method of claim 17, wherein the direction of preferential
orientation is defined by the flow of solvent during removal of the
solvent at a surface of the solution.
21. The method of claim 10, wherein the concentration of the
nanoparticles in the solvent is at least 5 mg/ml.
22. The method of claim 10, wherein the solution has a lower
surface tension than the pure solvent due to the presence of the
ligands in the solution.
23. The method of claim 10, wherein the ligands are one or more of
octylamine, butylamine, hexylamine, and any other alkylamine.
24. The method of claim 10, wherein the ligands are bound to the
surface of the nanoparticles by a chelating bond.
25. The method of claim 10, wherein the solvent comprises a mixture
of solvents.
26. The method of claim 25, wherein said mixture comprises a polar
and a nonpolar solvent.
27. The method of claim 26, where said mixture comprises an alcohol
and an organic solvent.
28. The method of claim 10, wherein, subsequent to causing the
nanoparticles to be deposited from solution, the film is heated so
as to remove the ligands.
29. The method of claim 28, wherein said removal of ligands occurs
by heating the film at a temperature less than 250.degree. C.
30. The method of claim 28, wherein said heating step is induced by
thermal annealing or by irradiation with light absorbed by the
nanoparticles.
31. The method of claim 28, wherein, subsequent to removing the
ligands, the film is immersed in a growth solution of nanoparticles
in a solvent.
32. The method of claim 31, wherein the growth solution is a
hydrothermal growth solution.
33. The method of claim 32, wherein the hydrothermal growth
solution is an aqueous solution comprising zinc nitrate and
ethylenediamine.
34. The method of claim 31, wherein the growth solution is heated
at a temperature below the bulk melting point of the nanoparticle
material so as to cause at least some of the nanoparticles to fuse
together.
35. The method of claim 34, wherein the temperature to which the
growth solution is heated is less than 100.degree. C.
36. The method of claim 31, wherein, subsequent to immersing the
film in a growth solution, the film is heated so as to cause
annealing of the nanoparticle film.
37. The method of claim 36, wherein the film is heated under an
atmosphere predominantly comprising nitrogen and hydrogen
gases.
38. The method of claim 36, wherein the annealing temperature to
which the film is heated is less than 250.degree. C.
39. The method of claim 10, wherein the nanoparticles are deposited
as a continuous film by means of one of spin coating, drop coating,
blade coating and microgravure coating, or as a patterned but
locally continuous film by direct printing.
40. The method of claim 10, wherein the nanoparticles are
semiconducting.
41. The method of claim 40, wherein said film of nanoparticles
forms part of the active layer of an electronic device.
42. The method of claim 41, wherein said electronic device further
comprises first and second contacts defining a current transport
path through the semiconducting layer extending therebetween, the
nanoparticles being generally aligned along the direction of the
current transport path.
43. The method of claim 41, wherein said electronic device is an
electronic switching device.
44. The method of claim 41, wherein said electronic device is
diode, such as a light-emitting, light-sensing or photovoltaic
diode.
45. The method of claim 42, wherein said elongate nanoparticles are
aligned with their long dimension preferentially oriented in the
plane of the substrate.
46. The method of claim 42, wherein said elongate nanoparticles are
aligned with their long dimension preferentially oriented normal to
the plane of the substrate.
47. The method of claim 41, wherein the mobility of the active
semiconducting layer is at least 0.5 cm.sup.2V.sup.-1s.sup.-1.
48. The method of claim 42, wherein the distance between the first
and second contacts defines a channel length and the ratio of the
channel length to the average length of the longer dimension of the
nanoparticles of the layer is larger than 1.
49. The method of claim 10, wherein the removal of the solvent
occurs when the dispersion is in contact with the substrate.
50. A method for fabricating a film of nanoparticles, the method
comprising: forming a dispersion of elongate nanoparticles in a
solvent, the nanoparticles having a longer dimension and a shorter
dimension and having one or more ligand molecules attached to their
surface; and causing the nanoparticles to be deposited from the
dispersion by removal of the solvent at a surface of the
dispersion; wherein the ligand molecules are selected so as to
cause the concentration of nanoparticles to be higher at the
surface of the solution than in the bulk of the solution, and the
shape of the nanoparticles is selected so as to promote mutual
alignment of the nanoparticles.
51. An active semiconducting layer that comprises inorganic
semiconducting elongate nanoparticles having a longer dimension and
a shorter dimension, the average ratio of the length of the longer
dimension to the length of the shorter dimension for the
nanoparticles of the layer being in the range 2 to 50 and the
average length of the longer dimension of the nanoparticles of the
layer being less than 1000 nm, wherein the nanoparticles of the
layer are generally mutually aligned.
52. The active semiconducting layer of claim 51, wherein the
nanoparticles are uniaxially aligned within domains having a
diameter greater than 1000 nm.
53. The active semiconducting layer of claim 51, wherein at least
some of the nanoparticles are fused together.
54. The active semiconducting layer of claim 51, wherein the
nanoparticles are zinc oxide nanoparticles.
55. The active semiconducting layer of claim 51, wherein the
mobility of the semiconducting layer is at least 0.5
cm.sup.2V.sup.-1s.sup.-1.
Description
[0001] There is significant interest in realizing high-performance
thin-film transistors (TFTs) based on solution-processible
semiconducting materials for applications requiring low-cost,
low-temperature manufacturing on large-area, flexible
substrates.[1,2] Much effort has been devoted to low-temperature,
solution processible organic semiconductors as a potential
alternative to traditional inorganic semiconductors. OTFTs with
mobilities of 0.01-0.1 cm.sup.2/Vs, good reliability, stability,
and device-to-device uniformity have been demonstrated.[3, 8] There
are also various approaches to realizing solution-processible
inorganic semiconductors, which provide a potential route to
significantly higher mobilities, but for which control of
electronic defect states when processed from solution at low
temperatures can be challenging.[9] Inorganic semiconductors might
also provide a route to high performance n-type TFTs required for
complementary circuits, which are traditionally difficult to
realize with organic TFTs although much progress has been made
recently.[10, 11]
[0002] A variety of solution-processible inorganic semiconductors
for TFTs have been reported.[9] These include tin(II) iodide based
organic-inorganic hybrids[12], chalcogenide semiconductors[13],
.mu.m-long semiconductor nanowires[14] and spherical
nanocrystals[15]. The semiconductor nanowire/nanocrystal approach
is very promising, since it allows to decouple the high-temperature
growth/synthesis of the nanowire from the low-temperature device
fabrication process, and achieve high performance.
[0003] Duan et al.[14] have used .mu.m-long semiconductor nanowires
as the active layer of TFTs. The long nanowires have a length
comparable to the channel length of the TFT and as a result yield
very high electrical performance, but are difficult to disperse
into a stable solution. They can be deposited and aligned by
sophisticated deposition techniques such as flow in capillaries and
Langmuir-Blogett techniques, but they do not form stable
dispersions with sufficiently long solution shelf lifetime, and as
a result are difficult to deposit over large areas, but standard
techniques such as printing, spin-coating, or other large-area
coating techniques. Another problem with semiconductor nanowires is
the low density of nanowires on the substrate. The typical distance
between the nanowires in the film is much larger than the nanowire
diameter, and the surface coverage is typically less than 20%.
[0004] Ridley et al.[15] have used spherical colloidal nanocrystals
of CdSe, which can be drop-cast onto a substrate and can re-melt to
form a uniform film after annealing at 350.degree. C. due to
lowering of the melting point for these ultra-small
nanocrystals.[16] However, the mobility of a thin film of spherical
nanocrystals is significantly lower than the maximum achievable
bulk mobility of the semiconducting material due to a large number
of grain boundaries in the sintered nanocrystal network. This is
because the nanoparticle diameter needs to be kept small (typically
less than 10 nm) in order to be able to sinter the particles by the
surface melting mechanism. Another problem with the spherical
colloidal nanoparticle approach is that it requires still
relatively high annealing temperatures for the sintering process.
CdSe nanocrystal devices shows n-type behaviour with mobility of 1
cm.sup.2V.sup.-1s.sup.-1 and on/off ratio of 3.1.times.10.sup.4
after annealing at 350.degree. C.
[0005] Some of the important requirements for using semiconductor
colloidal nanocrystals in this application include a good
dispersing capacity (>50 mg/ml) and adequate stability of the
dispersion (at least one week). Although there are many reports to
synthesize different kinds of nanocrystals, there is a need for
colloidal nanocrystal systems which can meet these
requirements.
[0006] Zinc oxide (ZnO) is an environmentally friendly transparent
semiconductor with a large band gap of 3.37 eV. TFT devices based
on polycrystalline ZnO as active layer have been reported with
mobility of around 0.2-3 cm.sup.2V.sup.-1s.sup.-1.[17, 18-21] Most
fabrication methods use a sputtering process to grow ZnO films.
Solution-processing techniques have also been used to fabricate ZnO
devices, but have suffered from poor device performance[20] or the
need to use high annealing temperature (700.degree. C.).[21] ZnO
nanospheres can be dispersed at high concentration beyond 75 mg/ml
for solar cell applications as shown in recent reports.[22, 23]
[0007] The shape of ZnO and other nanocrystals can be controlled
from nanosphere to nanorods by adjusting the growth time.[24]
Compared to the more commonly studied spherical colloidal
nanocrystals nanorods have an elongated cylindrical shape with
typical diameters of 3-10 nm and length of 10-100 nm.
[0008] The assembly of colloidal nanorods has been studied by Li et
al.[25,26] They used drop-casting to prepare nanorod crystals onto
an electron-microscopy grid, and also used capillaries to study the
formation of a lyotropic liquid crystalline phase of the nanorod
solution. However, they did not report formation of large-area,
continuous films of nanorods, nor did they incorporate the nanorods
into electronic devices.
[0009] The present specification discloses a method for improving
the performance and field-effect mobility of a TFT based on a thin;
solution-deposited film of an inorganic nanoparticle-based
semiconducting material.
[0010] According to an aspect of the present invention there is
provided an electronic switching device having a semiconducting
layer that comprises inorganic semiconducting elongate
nanoparticles having a longer dimension and a shorter dimension,
the average ratio of the length of the longer dimension to the
length of the shorter dimension for the nanoparticles of the layer
being in the range 2 to 50 and the average length of the longer
dimension of the nanoparticles of the layer being less than 1000
nm, wherein the nanoparticles of the layer are generally mutually
aligned.
[0011] Preferably the electronic switching device further comprises
first and second contacts defining a current transport path through
the semiconducting layer extending there between, the nanoparticles
being generally aligned along the direction of the current
transport path.
[0012] Preferably the distance between the first and second
contacts defines a channel length and the ratio of the channel
length to the average length of the longer dimension of the
nanoparticles of the layer is larger than 1.
[0013] Preferably the nanoparticles are uniaxially aligned within
one or more domains having a diameter greater than 1000 nm.
[0014] Preferably at least some of the nanoparticles are fused
together.
[0015] Suitably the nanoparticles are semiconducting oxide
nanoparticles. Many oxide semiconductors exhibit stable surface
composition when exposed to atmosphere and moisture, and do not
oxidize when exposed to air and moisture during growth or film
deposition. Examples of suitable oxide semiconductors include zinc
oxide, tin oxide, zinc tin oxide, indium oxide, zinc indium oxide,
or indium gallium zinc oxide nanoparticles. Provided that care is
taken to avoid surface oxidation also other, more reactive
semiconductors such as, but not limited to, Si, Ge, SiGe, GaAs,
InP, InAs, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, PbS,
PbSe, PbTe, SnS.sub.2, SnSe.sub.2, SnTe.sub.2 can be used.
[0016] Preferably the mobility of the semiconducting layer is at
least 0.5 cm.sup.2V.sup.-1s.sup.-1.
[0017] Preferably the density of the film is at least 50%, more
preferably more than 80%, of the bulk density of the inorganic
material of which the nanoparticles are comprised.
[0018] According to an aspect of the present invention there is
provided a method for fabricating a film of nanoparticles on a
substrate, the method comprising: forming a dispersion of elongate
inorganic nanoparticles in a solvent, the nanoparticles having one
or more ligand molecules attached to their surface, the
nanoparticles having a longer dimension and a shorter dimension,
and the ligand molecules including a functional group that enhances
the stability of the dispersion of the nanoparticles in the
solvent; and causing the nanoparticles to be deposited onto the
substrate from the dispersion by removal of the solvent at a
surface of the dispersion.
[0019] Preferably the ligands are organic or partly organic.
[0020] Suitably the semiconducting nanoparticles comprise an oxide
semiconductor. The oxide semiconductor may be zinc oxide, tin
oxide, zinc tin oxide, indium oxide, zinc indium oxide or indium
gallium zinc oxide.
[0021] Preferably the ligands are selected so as to cause the
concentration of nanoparticles to be higher at the surface of the
solution than in the bulk of the solution, and the shape of the
nanoparticles is selected so as to promote mutual alignment of the
nanoparticles.
[0022] Preferably the ratio of the length of the longer dimension
to the length of the shorter dimension for the nanoparticles in
solution is in the range 2 to 50 and the average length of the
longer dimension of the nanoparticles in solution is less than 1000
nm.
[0023] Preferably the solvent is removed at a surface of the
solution in such a way so as to define a direction of preferential
orientation for the nanoparticles and cause the nanoparticles to
become at least partially aligned along that direction.
[0024] Suitably the film defines a geometric plane and the
direction is out of the geometric plane defined by the film.
Alternatively the direction lies in the geometric plane defined by
the film.
[0025] Preferably the direction of preferential orientation is
defined by the flow of solvent during removal of the solvent at a
surface of the solution.
[0026] Preferably the concentration of the nanoparticles in the
solvent is at least 5 mg/ml.
[0027] Preferably the solution has a lower surface tension than the
pure solvent due to the presence of the ligands in the
solution.
[0028] Suitably the ligands are one or more of octylamine,
butylamine, hexylamine, and any other alkylamine. The ligands may
be bound to the surface of the nanoparticles by a chelating
bond.
[0029] Suitably the solvent is a mixture of solvents, such as a
mixture of a polar solvent which exhibits a favourable interaction
with the bare surface of the nanoparticles and a non-polar solvent
which exhibits a favourable interaction with the ligand molecule.
Suitably, the solvent is a mixture of an alcohol solvent and an
organic solvent, such as a mixture of chloroform and methanol.
[0030] Preferably, subsequent to causing the nanoparticles to be
deposited from solution, the film is heated so as to remove the
ligands. Preferably said removal of ligands occurs by heating the
film at a temperature less than 250.degree. C. Said heating step
may be induced by thermal annealing or by irradiation with light
absorbed by the nanoparticles.
[0031] Preferably, subsequent to removing the ligands, the film is
immersed in a growth solution of nanoparticles in a solvent.
Preferably the growth solution is a hydrothermal growth solution.
Suitably the hydrothermal growth solution is an aqueous solution
comprising zinc nitrate and ethylenediamine.
[0032] Preferably the growth solution is heated at a temperature
below the bulk melting point of the nanoparticle material so as to
cause at least some of the nanoparticles to fuse together. Suitably
the temperature to which the film is heated is less than
100.degree. C.
[0033] Preferably, subsequent to immersing the film in a growth
solution, the film is heated so as to cause annealing of the
nanoparticle film. Suitably, subsequent to immersing the film in a
growth solution, the film is heated so as to cause annealing of the
nanoparticle film Suitably the film is heated under an atmosphere
predominantly comprising nitrogen and hydrogen gases. Suitably the
annealing temperature to which the film is heated is less than
250.degree. C.
[0034] Suitably the nanoparticles are deposited by means of one of
spin coating, drop coating and blade coating, or by a direct
printing process such as inkjet printing, offset printing, gravure
printing, flexographic printing or screen printing. The
nanoparticles may be deposited in a continuous or as a patterned
but locally continuous film.
[0035] Suitably the nanoparticles are semiconducting. Suitably the
nanoparticles are zinc oxide nanoparticles.
[0036] Said film of nanoparticles may form part of the active layer
of an electronic device. The electronic device may further
comprises first and second contacts defining a current transport
path through the semiconducting layer extending therebetween, the
nanoparticles being generally aligned along the direction of the
current transport path.
[0037] Said electronic device may be an electronic switching
device. Said electronic device may be a diode, such as a
light-emitting, light-sensing or photovoltaic diode.
[0038] Suitably the elongate nanoparticles are aligned with their
long dimension preferentially oriented in the plane of the
substrate. Alternatively the elongate nanoparticles may be aligned
with their long dimension preferentially oriented normal to the
plane of the substrate.
[0039] Preferably the field-effect mobility of the active
semiconducting layer is at least 0.5 cm.sup.2V.sup.-1s.sup.-1.
[0040] Preferably the distance between the first and second
contacts defines a channel length and the ratio of the channel
length to the average length of the longer dimension of the
nanoparticles of the layer is larger than 1.
[0041] Preferably the removal of the solvent occurs when the
dispersion is in contact with the substrate.
[0042] According to an aspect of the present invention there is
provided a method for fabricating a film of nanoparticles, the
method comprising: forming a dispersion of elongate nanoparticles
in a solvent, the nanoparticles having a longer dimension and a
shorter dimension and having one or more ligand molecules attached
to their surface; and causing the nanoparticles to be deposited
from the dispersion by removal of the solvent at a surface of the
dispersion; wherein the ligand molecules are selected so as to
cause the concentration of nanoparticles to be higher at the
surface of the solution than in the bulk of the solution, and the
shape of the nanoparticles is selected so as to promote mutual
alignment of the nanoparticles.
[0043] According to an aspect of the present invention there is
provided an active semiconducting layer that comprises inorganic
semiconducting elongate nanoparticles having a longer dimension and
a shorter dimension, the average ratio of the length of the longer
dimension to the length of the shorter dimension for the
nanoparticles of the layer being in the range 2 to 50 and the
average length of the longer dimension of the nanoparticles of the
layer being less than 1000 nm, wherein the nanoparticles of the
layer are generally mutually aligned.
[0044] Preferably the nanoparticles are uniaxially aligned within
domains having a diameter greater than 1000 nm.
[0045] Preferably at least some of the nanoparticles are fused
together.
[0046] Suitably the nanoparticles are zinc oxide nanoparticles.
[0047] Preferably the mobility of the semiconducting layer is at
least 0.5 cm.sup.2V.sup.-1.
[0048] According to a first aspect of the present invention an
electronic switching device is disclosed, wherein the active
semiconducting layer is formed from inorganic colloidal
semiconducting nanorods deposited from liquid phase. Compared to
spherical colloidal crystals nanorods allow achieving better device
performance because of a smaller number of grain boundaries in the
film.
[0049] Compared to long nanowires, which typically have a length
exceeding 1 .mu.m, and do not allow formation of stable
dispersions, the nanorods can be formulated into stable dispersion
with concentrations exceeding 10 mg/ml and processed from solution
by techniques such as spin-coating, or other common large-area
coating techniques.
[0050] To obtain a stable nanoparticle dispersion the long
dimension of the nanorods L (for example, in the case of a
cylindrical nanorod L is the length of the rod) is preferably less
than 1 .mu.m, more preferably less than 500 nm, most preferably
less than 300 nm. These dimensions may refer to the size of each
nanorod, or to the average size of all nanorods in the
dispersion.
[0051] Preferably, the ratio between the small dimension of the
nanorod D (for example, in the case of a cylindrical nanorod D is
the diameter of the rod) and the long dimension of the nanorod is
in the following range: 2<L/D<50. More preferably, the ratio
L/D is in the range 5<L/D<20.
[0052] Preferably, the long (longer) dimension of the nanoparticles
is in range of 10 nm<L<300 nm, and the small (shorter)
dimension of the nanoparticles is in the range of 3 nm<D<50
nm. The longer dimension of a nanoparticle may be its longest
dimension. The shorter dimension of a nanoparticle may be its
shortest dimension. The shortest dimension is preferably transverse
to the longer dimension. Preferably the nanoparticle is
rod-shaped.
[0053] Where dimensions, ratios and figures are given herein for a
plurality of nanoparticles, such as those nanoparticles comprising
a semiconducting layer or film, those dimension, ratios and figures
should be taken to be averages over the relevant population of
nanoparticles (i.e. those nanoparticles actually comprising a
semiconducting layer or film). The average values may be mean,
median or modal average values.
[0054] Preferably, a ligand is attached to the surface of the
nanorod. The ligand is suitably an organic molecule (which could be
a partly organic molecule). Preferably it comprises a polar head
group which is able to form a bond with the surface of the
nanorods. The ligand also preferably comprises a functional group
which is soluble in the solvent used for the liquid phase
deposition. The ligand molecule may be soluble in the solvent.
Preferably, the bond between the ligand and the surface of the
nanorods is a weak bond such that it can be broken by
low-temperature annealing below typically 250.degree. C. or by
exposure to light and/or laser radiation. Preferably, the bond
between the ligand and the surface of the nanorod is a chelating
bond. Alternatively, the bond between the ligand and the surface is
a covalent bond.
[0055] Preferably, the nanorods are aligned such that their
long-axis is aligned preferentially in the plane of current flow in
the electronic switching device. More preferably, the nanorods are
aligned uniaxially such that their long-axis is aligned
preferentially along the direction of current flow in the
electronic switching device.
[0056] Preferably, the distance (preferably the typical or average
distance) between neighbouring nanorods in the as-deposited films
is less than the nanorod diameter, so that a dense film of nanorods
is formed. This facilitates the efficient transport of charges in
between nanorods. At the interface with the substrate the surface
coverage of nanorods is preferably higher than 80%, more preferably
higher than 90%. This is significantly higher than what can be
achieved with long nanowires[14], for which the spacing between
nanowires is more than 10 times the nanowire diameter, and the
surface coverage is typically less than 10%.
[0057] The density of the film may be at least 50%, and more
preferably more than 80%, of the bulk density of the inorganic
material of which the nanoparticles are comprised.
[0058] In comparison to the longer nanowires (length>1 .mu.m)
reported in the prior art[14], which allow spanning the gap between
two electrodes by a single nanowire, the use of nanorods according
to the present invention means that the channel length in a typical
device configuration with channel length of several .mu.m's is more
than 5-10 longer than the maximum dimension of the nanorods.
Nanorods allow retaining much better dispersion properties for
deposition from liquid phase and allow achieving dense, uniform
films, such that in spite of the large number of grain boundaries
in the channel high field-effect mobility of the transistor in
excess of 0.1 cm.sup.2/Vs can be achieved.
[0059] According to a second aspect of the present invention an
electronic switching device is disclosed wherein the active
semiconducting layer comprises inorganic semiconducting nanorods of
zinc oxide with a length between 10 nm and 1 .mu.m.
[0060] According to a third aspect of the present invention a
method is disclosed for preparing a uniform film of densely packed
nanorods by solution processing. By making use of a ligand that
lowers the surface tension of the nanorods uniform, densely packed
films of nanorods can be solution deposited by techniques such as
spin coating. In these films the nanorods can be made to adopt a
well defined orientation, such as in-plane or out-of plane
orientation of the long axis of the nanorods depending on process
conditions. In spin-coated films long ligands with lower surface
tension exhibit larger domain size and more pronounced in-plane
alignment of the nanorods resulting in significantly better FET
device performance than short ligands. The preferential orientation
of the nanorods can be influenced by the deposition process, and
the presence of a liquid flow during the deposition. In spin-coated
films preferential in-plane alignment of the nanorods can be
achieved while in drop-cast films with slow evaporation rate
nanorods are aligned preferentially normal to the substrate. In
films dried slowly between two substrates in-plane alignment was
found again with uniaxial nanorod alignment at the edges of the
drying film.
[0061] According to a fourth aspect of the present invention a
method is disclosed for forming an electronic switching device on a
substrate, wherein the method comprises depositing the active
semiconducting layer of the electronic switching device onto the
substrate from a liquid dispersion of colloidal nanorods, and
subsequently immersing the substrate into a growth solution to
increase the diameter and/or length of the nanorods on the
substrate.
[0062] Preferably, the as-deposited nanorods are aligned such that
their long-axis is aligned preferentially in the plane of current
flow in the electronic switching device, more preferably, the
as-deposited nanorods are aligned such that their long-axis is
aligned preferentially along the direction of current flow in the
electronic switching device.
[0063] According to a further aspect of the present invention said
growth solution is a hydrothermal growth solution.
[0064] The invention will now be described by way of example with
reference to the following figures:
[0065] FIG. 1 shows transmission electron microscopy images of ZnO
nanocrystals. (a) nanosphere with average diameter of 6 nm. (b)
nanorods with average length of 65 nm long and diameter of 10
nm.
[0066] Scheme 1 shows a schematic TFT device structure.
[0067] FIG. 2 shows log-linear scale plots of linear (V.sub.d=5V)
and saturated (V.sub.d=60V) transfer characteristics for as made
TFT device with (a) 6 nm nanospheres and (b) 10 nm.times.65 nm
nanorods without post-deposition hydrothermal growth. The devices
have been annealed at a temperature of 230.degree. C. before
measuring. The channel length (L) and width (W) are 20 .mu.m, and 1
cm, respectively. The capacitance value of the gate dielectric is
11.4 nF/cm.sup.2.
[0068] FIG. 3 (a) shows transfer characteristics of a device
composing of nanospheres in the linear region (V.sub.d=5V) and
saturated region (V.sub.d=60V) after post-deposition hydrothermal
growth. (b) and (c) Transfer characteristics and output
characteristics of a device made from nanorods after
post-deposition hydrothermal growth.
[0069] FIG. 4 shows scanning electron microscope images of a ZnO
film after post-deposition hydrothermal growth: (a) nanorods and
(b) nanosphere. The inset in FIG. 4(a) shows a scanning electron
microscope image of the as-spin-cast nanorod film. The inset in
FIG. 4(b) is an atomic force microscope topography of the
as-spin-cast nanosphere film. The scanning range is 1 .mu.m.times.1
.mu.m with z value of 10 nm. Figure (c) is a cross-sectional
scanning electron microscope image of a nanorod film after
post-deposition hydrothermal growth. Figure (d) is a plan-view
image of a film deposited from a dilute concentration of nanorods
after post-deposition hydrothermal growth.
[0070] FIG. 5 shows a schematic diagram of the most preferred
microstructure of the active semiconducting layer according to the
present invention. The layer is composed of uniaxially aligned
nanorods, and is shown before and after being subjected to a
hydrothermal growth step.
[0071] FIG. 6 shows photographs of water droplets on the surface of
(a) spin coated butylamine-ZnO film and (b) octylamine-ZnO film.
The left cartoon images show their responding nanorod surface
ligands (c) butylamine and (d) octylamine.
[0072] FIG. 7 shows polarized optical microscopy (POM) images of
(a) Butylamine-ZnO and (b) Octylamine-ZnO films prepared by
spin-coating. Image (c) and (d) are top view SEM image of
spin-coated ZnO films with (c) butylamine (d) octylamine
ligand.
[0073] FIG. 8 shows top view of SEM images of OCTA-ZnO film on
silicon oxide/silicon substrate processed under different
conditions. (a) Nanorod film fabricated drop-casting with slow (a)
and high (b) evaporation rate. (c) Nanorod film dried between a
glass and a Si/SiO.sub.2 wafer showing the edge of the film (c) and
the interior of the same film away from the edge (d). The inset in
(c) shows a large-area POM image of this film.
[0074] FIG. 9 shows (a) to (c) a schematic diagram illustrating the
different stages of spin-coating a solution of ZnO nanorods.
Initially, nanorods are mono-dispersed in the whole drop of
solution (a). When the substrate is rotating, the liquid film is
thinned and due to evaporation the solute concentration on the
surface is enhanced. (b). Nanorods can be aligned due to the radial
fluid flow. The orange arrows indicate the upward solvent flow due
to evaporation and replenishing of surface molecule. Green arrows
indicate the radial flow due to the rotation of the substrate. When
the solute concentration on the surface is high enough, the phase
transition from isotropic to mesomorphic phase will happen in (c).
(d) and (e) express that nanorods self assembly in dropping casting
process. Under slow evaporation the process is dominated by
vertical flow due to evaporation (d) leading to vertical alignment
of the nanorods (e). (f) shows schematically the situation for
solution drying between two substrates. (h) The nanorod diffusion
length on the surface is amplified exponentially by the surface
potential difference between nanorods in the interior bulk solvent
and at the liquid-air surface. The inset illustrates a nanorod on
the surface with length H penetrating above the surface of the
liquid.
[0075] FIG. 10 shows saturated (V.sub.d=60V) transfer
characteristics for as-made TFT fabricated by spin-coating of ZnO
nanorods with different ligands, solid line (OCTA-ZnO), dashed line
(BUTA-ZnO). (b) Output characteristics of a device made from
OCTA-ZnO) nanorods. The TFT device structure is shown in the inset
in (a) (bottom-gate with shadow mask evaporated Al top
contacts).
[0076] Table 1 shows TFT device characteristics of as-deposited ZnO
nanorod films, which were synthesized from different mole ratios
between potassium hydroxide and zinc acetate. Field effect
mobilities .mu..sub.sat were derived from the saturated region.
|V.sub.0| is the turn-on voltage of the TFTs.
EXAMPLE 1
Growth of ZnO Nanorods
[0077] ZnO nanorods are prepared according to a literature method
developed by Pacholski [23, 24] with some modification. 0.8182 g
(4.46 mmol) zinc acetate [Zn(Ac).sub.2] and 250 water was added
into a flask containing 42 ml methanol. The solution was heated to
60.degree. C. with magnetic stirring. 0.4859 g (7.22 mmol, purity
85%) potassium hydroxide (KOH) was dissolved into 23 ml methanol as
the stock solution which is dropped into the flask within 10-15
min. At a constant temperature of 60.degree. C. it takes 2 hours
and 15 minutes to obtain 6 nm diameter nanospheres. A small amount
of water was found helpful to increase the ZnO nanocrystal growth
rate. In order to grow the nanorods, the solution is condensed to
about 10 ml. This was found helpful before further heating to
decrease the growth time of the nanorods. Then it is reheated for
another five hours before stopping the heating and stirring. The
upper fraction of the solution is removed after 30 min. 50 ml
methanol is added to the solution and stirred for 5 min. The upper
fraction of the solution is discarded again after 30 min. This
process is repeated twice. For the second time washing, the upper
fraction of the solution is taken away after overnight staying.
Finally, 3.3 ml chloroform and 100 .mu.L n-butylamine are used to
disperse the nanorods. The nanorods concentration is about 85 mg/ml
and the solution is stable for more than two weeks. Using the
modified method reported here it only takes 5 hours to obtain 65 nm
long-nanorods instead of several days as reported in the
literature.[24]
[0078] Transmission electron microscopy images of ZnO nanospheres
(a) and nanorods (b) synthesized as above are shown in FIG. 1. The
diameter of the nanospheres is about 6 nm. The nanorods have an
average width of 10 nm and length of 65 nm. The nanorod length can
be tuned by the reaction time. However, long nanorods (longer than
100 nm) are quite difficult to disperse into any solution. In the
synthesis process, it is critical to have the correct mole ratio
between KOH and Zn(Ac).sub.2. The chemical composition of
as-prepared nanorods is determined by the initial mole ratio.
Variations in stoichiometry affect the conductivity of the films,
and the mobility and ON-OFF current ratio of the TFTs (see
below).
EXAMPLE 2
Comparison Between Nanorod and Nanosphere Based TFTs
[0079] Nanocrystal films and devices are fabricated on
SiO.sub.2(300 nm)/Si substrates with photolithographically
patterned interdigitated Cr(3 nm)/Au(12 nm) electrodes. The device
structure is shown in Scheme 1. Before spin-casting the ZnO
solution, the substrate is cleaned in an oxygen plasma at a power
of 150 W for 2 min. The film is spin-coated from filtered (0.45
.mu.m PTFE filter) ZnO solution with a speed of 2000 rpm. Then the
devices are annealed at 230.degree. C. in N.sub.2/H.sub.2(V/V,
95:5) for 30 min.
[0080] Characteristics for three devices made from ZnO nanorods
synthesized with different mole ratios are summarized in Table 1.
All devices exhibit n-type field-effect conduction. The optimized
mole ratio is 1.62. It is found that the conductivity increases and
mobility decreases as the stoichiometry is varied from the
experimentally determined optimum mole ration in both directions.
The stoichiometry can be characterized by X-ray diffraction. The
(002) diffraction signal of the ZnO nanocrystals only comes from
zinc atoms in the wurtzite crystal structure. It has been found
that the (002) signal of the ZnO nanocrystals in their X-ray
diffraction patterns is maximum if the mole ratio is near to its
stoichiometric value [27], which means that there will be the
lowest oxygen vacancy in this crystal structure at this ratio.
[0081] ZnO films containing a low concentration of oxygen vacancies
should exhibit low conductivity because oxygen vacancies behave as
deep donors.[28] Consistent with this expectation TFT devices based
on this ratio show the lowest conductivity. It is worth mentioning
that small variations in mole ratio do not appear to have a
significant effect on the shape and size of the nanorods, but do
greatly affect the TFT device performance. We believe that the
large difference of TFTs' characteristics originates in small
changes of the stoichiometry of the ZnO films.
[0082] To obtain reproducible TFT performance the formation of
high-quality films by techniques such as spin-coating is very
important. We have found that high-quality nanocrystal films can be
obtained from high concentration solutions (>50 mg/mL). The
as-prepared ZnO nanocrystals comprise acetate (CH.sub.3COO.sup.-)
ligand groups chelating with zinc atoms on the surface of
nanocrystals. The ligands are very important to facilitate the
dispersion of the nanocrystals in the solvent. For small
nanospheres (approximately 6 nm), it is quite easy to achieve a
high concentration solution using just the short acetate ligands.
However, the acetate ligands are not sufficient to achieve high
concentration dispersions of the longer nanorods. Alkylamine
ligands can be used as ligands to help ZnO nanocrystal
suspension.[27] When alkylamine is added to the solution, the
10.times.65 nm nanorods can be dispersed into chloroform with
concentration as high as 90 mg/ml. For the nanospheres there is a
large number of microcracks in the spin-cast films if butylamine is
not added to the solution. Alkylamine ligands can be removed from
the surface by low-temperature annealing at 200.degree. C. due to
their low boiling points (butylamine 78.degree. C.) and below, and
result in stable dispersion that do not aggregate or sediment for
periods of days. We have found that longer alkylamine ligands
(octylamine) lead to more stable dispersions than shorter ligands
(butylamine). With octylamine dispersions are stable over periods
of several weeks. In examples 2 and 3 we use butylamine as a
ligand. A comparison between different ligands will be presented in
examples 5, 6 and 7.
[0083] The as-prepared films are annealed under nitrogen/hydrogen
atmosphere to increase mobile carrier concentration and
field-effect mobility. It has been reported that hydrogen can be
incorporated into ZnO films in high concentration at annealing
temperatures of 200.degree. C. and behave as a shallow donor acting
as a source of conductivity.[28] At the same time, the ligand can
be partly removed due to this mild heating. The characteristics of
TFTs made from ZnO nanospheres (a) and nanorods (b) are shown in
FIG. 2. Both TFT devices exhibit clean n-type transistor behavior
with low turn-on voltage |V.sub.0|=0-8V and good operating
stability. For the device made from 6 nm nanospheres, the ON-OFF
ratio is 5.times.10.sup.3 and the linear and saturated field-effect
mobilities are 2.37.times.10.sup.-4 cm.sup.2V.sup.-1s.sup.-1, and
4.62.times.10.sup.-4 cm.sup.2V.sup.-1s.sup.-1, respectively. The
TFT device performance is significantly improved when nanorods are
used as the active layer instead of nanospheres. These devices
exhibit an ON-OFF ratio of 1.1.times.10.sup.5 and higher mobility
of 0.023 cm.sup.2V.sup.-1s.sup.-1 derived from the saturated
operating region and 0.013 cm.sup.2V.sup.-1s.sup.-1 derived from
the linear region. The mobility is improved by almost two orders
between a 6-nm nanosphere and a 65-nm-long nanorods. The
approximately 10.times. larger size of the nanorod particles
compared to the nanospheres will significantly reduce the number of
interparticle hopping events that an electron has to undergo when
moving from source to drain electrode (channel length L=20 .mu.m).
This will result in an increase of the mobility even if the
nanorods are not uniaxially aligned along the direction of current
flow. However, another important reason for the improved
performance of the nanorod device is believed to be related to the
favourable in-plane self-alignment of the colloidal nanorods when
spin-coated onto the substrate as discussed below (see FIG. 4
(a)).
EXAMPLE 3
Improvement of Mobility by Post-Deposition Hydrothermal Growth
[0084] The TFT device performances can be further enhanced by the
post-deposition hydrothermal growth step in aqueous solution. For
the additional hydrothermal growth, the substrates are immersed
upside-down into a glass beaker filled with an aqueous solution
containing zinc nitrate (0.025M) and ethylenediamine (0.04M) with
slow stirring at 90.degree. C. The devices are taken out after 50
min and rinsed with deionised water. Finally, the device is
annealed at 200.degree. C. for 15 min in N.sub.2/H.sub.2 atmosphere
after drying with nitrogen. Electric measurements are performed in
a nitrogen atmosphere to avoid any possible effect from oxygen and
humidity.
[0085] The corresponding TFT device characteristics are shown in
FIG. 3 (b). Devices composing of nanorods have achieved a mobility
of 0.61 cm.sup.2V.sup.-1s.sup.-1 derived from the saturated region
and on/off of 3.times.10.sup.5. This performance is comparable to
that of TFT devices fabricated in the same device structure by
sputtering methods (mobility: 1.2 cm.sup.2V.sup.-1s.sup.-1; on/off:
1.6.times.10.sup.6).[18] For comparison, single ZnO nanowire
transistors with mobility of 1-5 cm.sup.2V.sup.-1s.sup.-1 have been
reported.[30] For the solution-based method reported here the raw
materials and deposition methods are low-cost, and the aqueous
hydrothermal growth in an open vessel should be applicable to
large-area substrates.
[0086] To investigate the relationship between film microstructure
and device performance and to identify the mechanisms for the
observed improvements of device performance we have performed
scanning-electron microscopy (SEM) and atomic force microscopy
(AFM) (FIG. 4). From SEM and AFM images such as FIG. 4(a) it is
clear that in as-spun films the nanorods are preferentially
oriented with their long-axis in the substrate plane. The
interactions between the colloidal nanorods during solution growth
lead to the formation of small liquid crystalline-like domains with
a size on the order of 100 nm in which the nanorods are oriented
parallel to each other (see inset of FIG. 4a). Similar colloidal
self-organisation into nematic and smectic-A ordered solids has
been reported for CdSe and BaCrO.sub.4 nanorods.[25, 31-32] Due to
this self-alignment of the rods the probability of encountering
high-angle domain boundaries is reduced. We believe that this
oriented in-plane self-assembly of the colloidal nanorods is an
important factor contributing to the enhanced mobility of the
as-deposited nanorod films compared to nanosphere films.
[0087] During post-deposition hydrothermal growth, the nanorods
grow further along their c-axis forming longer rods, as shown in
FIG. 4(c) and (d). The average final nanorod length is as long as
300 nm. Cross-sectional SEM images of the nanorod film clearly show
that near the interface with the substrate the nanorods retain
their favorable in-plane orientation during the post-deposition
hydrothermal growth, while on the surface of the film an increasing
number of nanorods grow preferably normal to the film plane. We
also observe an increase in the diameter of the nanorods, which
appears to be mainly occurring due to fusing of several nanorods
(FIG. 4(d)). Similar increase in nanorod size has also been
observed in vertically oriented ZnO arrays where microwires are
formed by fusing of many nanowires.[33,34] The diameter of
individual nanorods also increases slightly to about 15 nm after
hydrothermal growth, but by fusing of several closely-packed
nanorods the diameter can become as large as approximately 60 nm.
Generally, the fusing process prefers to take place in the densely
packed regions of the films in which nanorods are oriented parallel
to each other. This increase in nanorod diameter and length is
responsible for the observed improvement of TFT device performance
after post-deposition hydrothermal growth. Here, we use a two-step
approach to obtain self-aligned 300-nm-long nanorods. It is quite
difficult to achieve this in a single step due to the poor
dispersion properties of long nanorods or nanowires.
[0088] If the film is made from nanospheres and subjected to
post-deposition hydrothermal growth the TFTs show only a small
improvement of mobility to 0.0024 cm.sup.2V.sup.-1s.sup.-1 and
on/off ratio of 5.times.10.sup.4. The mobility is more than two
orders of magnitude lower than that of TFTs made from nanorods by
the same fabrication process. During post-deposition hydrothermal
growth the as-spin-cast nanosphere-seed film can grow into an array
of ZnO wires, which are, however, aligned randomly with respect to
the substrate normal.[33] This is less favourable than the in-plane
orientation of nanowires obtained near the substrate interface in
films deposited from a nanorod dispersion. The nanosphere films
exhibit good uniformity, as shown in FIG. 4(b). When immersed in
the aqueous solution, the nanospheres grow along a random
direction. Some rods are perpendicular to the substrate and appear
as bright spots of high electron density in the SEM image; and some
rods are growing at an angle to the substrate normal achieving a
limited length of about 50 nm. Although this length is comparable
to that of the as-prepared nanorod films (65 nm), the TFTs made
from nanosphere films subjected to hydrothermal growth exhibit
about one order of magnitude lower mobility than those made from
as-prepared nanorod films. This is further evidence that the
alignment of the nanorods is an important factor responsible for
the improved performance of devices made from nanorods.
EXAMPLE 4
Uniaxial Liquid-Crystalline Alignment of Active Semiconductor
Nanorod Layer Followed by Post-Deposition Hydrothermal Growth
[0089] The above observations show the beneficial effects on the
device performance, and charge carrier mobility if the long axis of
the nanorods is aligned in the plane of current transport. Nanorods
of the type used here have been shown to form nematic as well as
smectic liquid crystalline phases, in which the nanorods are
aligned uniaxially with their long axis parallel to each other in
micron size domains.[25,26] The liquid--crystalline properties of
the nanorods can be used to produce uniaxially aligned films of
nanorods in which the long nanorod axis is aligned along the
direction of current flow in the device (FIG. 5). The structure can
be either smectic or nematic, preferably smectic. In this structure
optimum use is made of the fast charge transport along the long
axis of the nanorod. The probability for encountering transport
impeding high-angle grain boundaries is reduced. During the
subsequent hydrothermal growth step the growth of larger nanowires
is facilitated due to merging of several small nanorods into a
larger nanowire. In this way arrays of long and aligned nanowires
with a length comparable of longer than the channel length of the
transistor can be grown. Note, that according to the present
invention when the nanorods are deposited their length is less than
1 .mu.m. As a consequence, they exhibit good dispersion properties.
The network of aligned nanowires achieved after hydrothermal growth
can have much larger dimensions, which would not have allowed good
and stable dispersions for liquid-phase deposition.
[0090] To align the as-deposited nanorods uniaxially a range of
techniques known from the field of liquid-crystalline anisotropic
organic molecules (as used for example, in active matrix liquid
crystal displays) can be used. These include, but are not limited
to, use of alignment layers with either an aligned molecular
structure (such as polyamide alignment layers produced by
mechanical rubbing), or aligned topographic structures/grooves, in
which the nanorods align along the walls of the grooves during
evaporation of the solvent. Alternatively, alignment by exposure to
electrical or magnetic fields, polarised light, or through shear,
flow or capillary force alignment can be used. The nanorods are
formulated preferably in a solvent, in which they exhibit lyotropic
behaviour, i.e. in which beyond a certain concentration of
nanorods, the solution becomes liquid-crystalline at a temperature
near room temperature. The concentration threshold for exhibiting
lyotropic properties might be reached in the original solution, or
maybe crossed by drying of solvent during deposition of the
solution.
[0091] As shown in below the nanorods can also be aligned by
selecting a ligand that lowers the surface tension of the nanorods
and acts as a surfactant in the solvent from which the nanorods are
deposited.
EXAMPLE 5
Dependence of Surface Tension of Nanoparticles on Ligand Length
[0092] The solution self-assembly of semiconducting ZnO nanorods
can be sensitively controlled by the surface chemistry of the
nanorods, in particular, by tuning the length of the ligand
attached to the surface of the nanorods. The use of longer ligands
with lower surface tension or ligands with a chemical structure
that lowers the surface tension helps the formation of a
well-ordered mesophase as observed by polarized microscopy and
scanning electron microscopy.
[0093] To keep the nanoparticles well dispersed in an organic
solvent an organic monolayer ligand is attached to the surface of
the inorganic nanoparticle. For the ZnO nanorods acetate group on
the surface of ZnO nanorods can be used during the initial
synthesis of the nanoparticles. However, acetate group have a short
chain length and can result in the nanorods being negatively
charged. To make the nanorods to be dispersed in other organic
solvents, butylamine (BUTA) was used as ligand. BUTA is attached on
the ZnO surface by an interaction between the nitrogen atom of BUTA
and the zinc atom of ZnO.
[0094] An attractive feature of this ligand is that it can be
removed easily by low temperature annealing due to its neutral
charge property and low boiling point. This is important for
achieving efficient charge transport between the nanorods. In the
present work we have used systematically alkylamine ligands with
different chain length to tune the surface chemistry of the ZnO
nanorods, and study the effect of the different ligand length on
the self assembly properties of the nanorods.
[0095] We have found that the use of different ligand lengths
results in a significant variation of the surface tension of the
nanoparticles. A higher water contact angle is observed for nanorod
films with longer carbon ligand. As shown in FIG. 6 for octylamine
(OCTA) modified ZnO, a water contact angle of 85.0.degree. is
observed compared to 74.4.degree. (water) for BUTA-ZnO.
[0096] The particle-air surface tensions .gamma..sub.p/a for
different ligands are estimated from the measured water contact
angles of films of nanoparticles. To do this we make use of a
theoretical model [Good, R. J.; Girifalco, L. A. J. Phys. Chem.
1960, 64, 561-565] for estimating the liquid-particle
(water/ligand) interfacial tension .gamma..sub.t/p from the surface
tension of the particles .gamma..sub.p/a and the liquid
.gamma..sub.l/a:
.gamma..sub.l/p'.gamma..sub.p/a+.sub..gamma..sub.l/a-2.PHI.(.gamma..sub.-
p/a/.gamma..sub.l/a).sup.1/2 (1)
[0097] By using equation (1) and Young's equation,
.gamma..sub.p/a=.gamma..sub.l/a cos .theta.+.gamma..sub.l/p,
.gamma..sub.l/p can be eliminated and using the known liquid-air
surface tension .gamma..sub.l/a for water .gamma..sub.p/a can be
expressed as:
.gamma..sub.p/a.gamma..gamma..sub.l/a(1+cos
.theta.).sup.2/(4.PHI..sup.2) (2)
[0098] The parameter .PHI. can be shown to be determined by the
molar volume of the particle V.sub.p and liquid V.sub.1:
.PHI.=4(V.sub.pV.sub.l).sup.1/3/(V.sub.p.sup.1/3+V.sub.1.sup.1/3).sup.2
(3)
[0099] The parameter can be obtained if the particle molar volume
V.sub.p values is estimated from the corresponding ligand
alkylamine molar volume because the effective interface is that
between the liquid and the ligand molecules on the surface.[1] The
parameter .PHI..sub.water/BUTA value of 0.924 is obtained according
equation (3) to determine interfacial energy between water and
BUTA-ZnO. In the same way, .PHI..sub.water/OCTA value is 0.875 for
that between water and OCTA-ZnO. We can estimate .PHI..sub.p/a for
different ligands using Equation 2 and the known surface tension of
71.98 mJ/m.sup.2 of water. We find that there is a difference of
6.16 mJ/m.sup.2 between the surface tension of BUTA-ZnO
(.gamma..sub.p/a.sup.BUTA=33.94 mJ/m.sup.2) and that of OCTA-ZnO
(.gamma..sub.p/a.sup.OCTA=27.78 mJ/m.sup.2).
[0100] Using these values for the surface tension of the ligand
modified nanorods we then can apply Equation 1 again to the case of
the nanorod solution in chloroform/methanol and calculate the
interface tension between the nanorods and the solvent using the
appropriate values of .PHI. for chloroform/methanol as the solvent.
Here the liquid is chloroform/methanol and its surface energy
.gamma..sub.l/a is 26.74 mJ/m.sup.2 which is in between the surface
energy of methanol .gamma..sub.methanol/air=22.98 mJ/m.sup.2 and
that of chloroform .gamma..sub.chloroform/air=28.02 mJ/m.sup.2 as
determined by the ring method. We have not been able to observe
surfactant action of the alkylamine molecule in the
chloroform/methanol solvent, i.e., no lowering of the solvent
surface tension compared to that of the pure solvent was observed
when alkylamine molecules were dissolved at a concentration of
1.5%, V/V in chloroform/methanol. The interfacial free energy value
for the chloroform/methanol-BUTA-ZnO interface is
.gamma..sub.p/l.sup.BUTA=0.670 mJ/m.sup.2 based on the parameter
.PHI..sub.solvent/BUTA0.996. Similarly, the interfacial free energy
from chloroform/methanol-OCTA-ZnO (.gamma..sub.p/l.sup.oct) is
.gamma..sub.p/l.sup.OCTA=1.21 mJ/m.sup.2 for
.PHI..sub.solvent/OCTA=0.978.
EXAMPLE 6
Dependence of Solution Self-Assembly on Choice of Ligand and Method
of Solution Deposition
[0101] We have investigated the nanorod solution self assembly for
different film deposition techniques as a function of ligand
length, and have compared in particular spin-coating and
drop-casting. ZnO nanorods with alkylamine ligands can be well
dispersed in chloroform/methanol solvent with high concentration
(up to 50 mg/ml) enabling formation of uniform thin films by spin
coating. When solutions of ZnO nanorods with different ligand
lengths are spin-coated on the substrate, the resulting films
exhibit a strong variation of microstructure depending on the
length of the ligand. In OCTA-ZnO films with low surface free
energy clear optical contrast is observed in POM images with
crossed polarizers indicating the presence of large crystalline
domains with uniaxial alignment of the nanorods on a length scale
of several 1-10 .mu.ms (FIG. 7(b)). In contrast in BUTA-ZnO films
the crossed-polarizer images are uniformly black suggesting that
the film is more optically isotropic, i.e., that crystalline order
in the film is either absent or occurs on a submicrometer length
scale that is too short to be observable by POM (FIG. 7(a)). In
hexylamine(HEXA) modified ZnO intermediate behaviour with optical
contrast somewhat weaker and occurring on a shorter lengths scale
than in films of OCTA-ZnO was observed (not shown).
[0102] The origin of the optical contrast in the POM image was
investigated by SEM. The images show clearly that using the method
according to the present invention it is possible to form uniform,
densely packed films of nanorods by simple solution coating
techniques such as spin-coating. In all spin-coated films we
observed preferential in-plane alignment of the long-axis of the
nanorods (FIGS. 7 (c) and (d)). However, in the OCTA-ZnO films
large domains are present in which the nanorods are aligned
uniaxially parallel to each other in the plane of the film. Only a
few domain boundaries are visible in the SEM image of FIG. 7(d). In
contrast, in the BUTA-ZnO case (FIG. 7(c)) although the nanorods
are also aligned preferentially in the plane of the film, only much
smaller domains are present, and over a length scale of 1 .mu.m the
orientation of the nanorods appears isotropic. The SEM observations
explain the optical contrast observed in POM, and show clearly that
the ligand length has an important influence on the ability of the
nanorods to form large-scale uniaxially oriented
microstructures.
[0103] Different behaviour was observed in the case of
drop-casting. When a droplet of nanorod OCTA-ZnO solution was dried
on the substrate with a slow evaporating rate we observed
preferential out-of-plane alignment of the nanorods in contrast to
the in-plane alignment observed in spin-coated films. In some areas
well-defined smectic ordering was observed that manifests itself in
well-defined steps on the surface of the film with a step height of
90 nm equal to the length of the nanorods (FIG. 8(a)). When the
films were drop-cast under fast evaporation conditions a more
disordered structure with more random orientation of the nanorods
resulted (FIG. 8(b)).
[0104] We also investigated the drying of a drop of nanorod
solution in between two substrates (bottom SiO.sub.2/Si wafer and
top quartz substrate), such that the free solution-air interface
was not present, and evaporation occurred at the edges of the
liquid film stacked between the two substrates. Under such
conditions a well ordered nanorod structure can be observed where
nanorods are again oriented parallel to the substrate as in
spin-coated films. In the vicinity of the edges of the film the
nanorods are aligned uniaxially perpendicular to the contact line.
In some areas clear evidence for smectic ordering was obtained, as
shown in FIG. 8(c). This well ordered assembly takes place at the
edge of the drying solution where solvent evaporation leads to a
liquid flow in the plane of the film perpendicular to the contact
line. POM images of a drop of OCTA-ZnO nanorods dried between the
two substrates show bright contrast at the edges of the film a
combined effect of the uniaxial, in-plane alignment of the nanorods
and the increased thickness of the film near the edge. The films
made in this way are not continuous though (see inset of FIG. 8(c).
In areas of the films which are more than a few turns away from the
contact line in-plane nanorod alignment without uniaxial alignment
along a preferred direction is observed (FIG. 8(d)). By comparing
substrates with and without surface treatment using self-assembled
nionolayers, such as HMDS, we found that the orientation and
assembly of the nanorods is not sensitive to the substrate surface
tension. These observations suggest that the liquid flow in the
drying solution and the presence of the liquid-air interface play
important roles in determining nanorod self assembly.
[0105] Without wanting to be bound by theory the self-assembly
behaviour is believed to be determined by the segregation and
nucleation of the nanoparticles to the liquid/air interface. The
driving force for nanorod segregation to the surface can be
estimated by calculating the change of surface potential
.DELTA..mu. when a nanorods segregates from the bulk of the liquid
to the surface. For the purpose of this estimate we assume that the
nanorods on the surface are oriented vertical to the surface due to
vertical liquid flowing, and that a length H of the long axis of
the nanorods (total length L, radius R) is protruding out of the
surface and is in contact with air (see Scheme 9(h)). There are
three contributions to the surface potential of a nanorod on the
surface measured with respect to that of the surface of the pure
liquid. [0106] a) Surface potential of nanorod/air interface:
[0106] .mu..sub.p/a=.gamma..sub.p/a(.pi.R.sup.2+2.pi.RH) (4) [0107]
b) Surface potential of nanorod/liquid interface:
[0107] .mu..sub.p/l=.gamma..sub.p/l[.pi.R.sup.2+2.pi.R(L-H)] (5)
[0108] c) Negative surface potential due to the portion of the
liquid/air interface that has been displaced by the nanorods:
[0108] .mu..sub.l/a=.gamma..sub.l/a.pi.R.sup.2 (6)
[0109] The total surface potential can be written:
.mu..sub.total=.pi.R.sup.2(.gamma..sub.p/a+.gamma..sub.P/l+.gamma..sub.l-
/a)+2.gamma..sub.p/l.pi.RL+H2.pi.R(.gamma..sub.p/a-.gamma..sub.p/l)
(7)
[0110] The liquid is chloroform/methanol in this case. The surface
potential .mu..sub.total will be a minimum value .mu..sub.min if
H=0, and only the end surface of the nanoparticle is exposed to air
since (.gamma..sub.p/a-.gamma..sub.p/l)>0:
.mu.min=.pi.R.sup.2(.gamma..sub.p/a+.gamma..sub.p/l-.gamma..sub.l/a)+2.g-
amma..sub.p/l.pi.RL=.gamma..sub.l/a[.pi.R.sup.2(x+y-1)+2y.pi.RL]
(8)
with x=.gamma..sub.p/a/.gamma..sub.l/a and
y=.gamma..sub.p/l/.gamma..sub.l/a. In comparison, the surface
potential .mu..sub.liquid of a nanorod totally immersed in liquid
is
.mu..sub.liquid=.gamma..sub.p/l(2.pi.R.sup.2+2.pi.RL) (9)
[0111] The potential barrier for nanoparticles to segregate from
the bulk liquid to the surface .DELTA..mu..sub.1 is
.DELTA..mu..sub.1=.mu..sub.liquid=.mu..sub.min=.pi.R.sup.2(.gamma..sub.p-
/l-.gamma..sub.p/a+.gamma..sub.l/a)=.pi.R.sup.2.gamma..sub.l/a(y=x+1)
(10)
[0112] Here, the average values for ZnO nanorods are R=5.5 nm and
L=92 nm and the difference in size due to nanorods ligands is
neglected. In the BUTA-ZnO case, since x=1.269 and y=0.0251,
.DELTA..mu..sub.1.sup.BUTA=-0.2447.pi.R.sup.2.gamma..sub.l/a is
obtained from Equation 10 respectively, i.e. there is no driving
force for the particle to expose a portion of its surface to air.
However, in the case of OCTA-ZnO, .DELTA..mu..sub.1.sup.OCTA=0.063
.pi.R.sup.2 .gamma..sub.l/a can be calculated using x=0.955 and
y=0.0433, and this constitutes a potential for trapping the
particle at the surface.
[0113] In this framework we can rationalize the observed assembly
and alignment of the nanorods under different deposition conditions
in the following way. We first consider the case of drop-casting at
a low evaporation rate. Due to evaporation solvent molecules move
towards the interface from the interior to replenish the surface
liquid and the surface moves towards the substrate. At the same
time nanorods diffuse in solution. Under the conditions used here
the diffusion velocity is slower than the velocity of the moving
solid-air surface, and therefore nanorods from the interior of the
solution are swept towards the surface. The flux of nanorods
impinging onto the surface is given by f=cv, where c and v are the
concentration and flow rate of nanorods with respect to the
surface, respectively. According to the Onsager-Flory rigid rod
model [37] at sufficiently low concentration of the rods in
solution, the rod orientation is isotropic. However, as the
concentration increases, it becomes increasingly difficult for the
rods to point in random directions and a concentrated solution of
nanorods will undergo a phase transition into a lyotropic liquid
crystalline phase in which the nanorods are aligned uniaxially
along a director. This phase transition is expected to occur at a
critical volume ratio of nanocrystals to solvent of
.PHI..sub.0=cD/L, where c is a constant with a value c=4.5 for the
Onsager model and c=12.5 for the Flory model. D is the rod diameter
and L its length. The colloidal nanorod solution is expected to
satisfy the assumptions made by the rigid rod model. [37]
[0114] A nanorod is considered to be trapped at the liquid-air
interface if .DELTA..mu..sub.1 is positive, and the surface
potential change .DELTA..mu..sub.1 is closely related to the
diffusion length .lamda. of the nanorods on the surface
.lamda..sup.2=2L.sup.2 exp[.DELTA..mu..sub.1/k.sub.bT] where
k.sub.b and T are Boltzmann's constant and temperature. For the
above values of .DELTA..mu..sub.1 this relationship shows that
small changes of surface potential change due to differences in
surface tension between different ligands can result in large
differences of diffusion length, as shown in Scheme 9(h). For the
BUTA-ZnO, the surface potential change value is negative, which
means the nanorods have to overcome a potential barrier from
interior bulk liquid to interface. This barrier will prevent the
nanorods from segregating towards the interface. In contrast, the
estimated surface potential change .DELTA..mu..sub.1.sup.OCTA is
positive, which means that a driving force exists that favors the
nanorod segregation towards the air-liquid interface. Using the
interfacial potential change .DELTA..mu..sub.1.sup.oct=3.89
k.sub.bT we estimate the diffusion length .lamda..sup.OCTA to be on
the order .about.0.91 .mu.m. A long diffusion length is closely
related to the critical concentration .phi..sub.0 for the
liquid-crystalline phase transition. A certain minimum flow rate
value f.sub.0 is needed to reach this concentration, which can be
estimated to be f.sub.0=4.phi..sub.0D.sub.diff/.lamda..sup.2, where
D.sub.diff is the diffusion constant at the liquid-air interface.
From this expression it is clear that nanorods with longer
diffusion length at the liquid-air interface are more likely to
reach the critical concentration to undergo nucleation on the
surface. This analysis suggests that the reason for the larger
domain size of the OCTA-ZnO films is related to the ability of the
OCTA-ZnO nanorods to undergo nucleation at the air-liquid
interface. In the case of nanorods which do not have a tendency to
segregate to the surface, such as BUTA-ZnO it is more likely that
nucleation occurs in the bulk of the liquid leading to poorer
alignment of the nanorods.
[0115] Within this framework once the lyotropic solution has formed
on the drying surface the orientation of the nanorods is expected
to be determined by flow alignment due to the solution flow on the
surface. This explains why in the case of drop casting the
preferred orientation of the nanorods is perpendicular to the
substrate since in this case the dominant solution flow is due to
evaporation and is normal to the surface (Scheme 9(d)). In the case
of the nanorod solution drying between two substrates (Scheme 9(f))
the nucleation is expected to occur at the edges of the drying film
where solvent can escape between the two substrates, and where the
concentration is highest. Here the solution flow is in the plane
perpendicular to the edge and this explains the strong uniaxial
alignment, and in some cases even smectic alignment of the rods
near the edge of the film. This observation also supports the
conclusion that under these conditions the transition from
isotropic phase to lyotropic phase happens preferentially at the
liquid-air interface where the concentration is highest and not in
the bulk interior of the solution.
[0116] The case of drop-casting with high evaporation rate (FIG.
8(b)) is more complex to explain. Here we observed a complex
alignment motive with small domains that adopt either in-plane,
out-of-plane or tilted alignment of the nanorods. This might be
related to a complex pattern of liquid flow on the surface. In this
situation one would expect to observe a normal component of the
flow due to replenishment of solvent evaporating at the surface, a
lateral flow along the substrate to transport liquid to the edges
of the film that have a higher evaporation rate (coffee-stain
effect). It is also possible that convective flow cells might be
formed driven by either a temperature gradient or a concentration
gradient on the surface caused by the evaporation of solvent. An
alternative explanation for the non-uniform alignment under such
process conditions is that nucleation occurs in the bulk of the
solution at a stage when the film is still thick enough, such that
capillary forces do not force in-plane alignment of the rod (see
discussion below).
[0117] The case of spin-coating is also complex. The factors that
control coating deposition in the spin coating process have been
extensively studied [38-42] and include the solution concentration,
density, viscosity, surface free energy, solution evaporating rate
and rotation speed. When a film is fabricated by spin casting from
nanorod solution with high solvent drying rate the liquid flow has
both an in-plane component due to rotation of the substrate as well
as an out-of plane component due to evaporation at the surface. The
spin coating process can be divided into several stages. In the
early stages the process is determined by the radial convection
flow due to the rotation of the substrate leading to initially
rapid thinning of the film while the solute concentration in the
liquid remains constant. The radial flow velocity is highest on the
surface and drops to zero at the interface with the solid
substrate. As the film thins the radial flow velocity and the film
thinning rate due to convection slow down, and evaporation of
solvent at the surface begins to make an increasingly important
contribution to the thinning of the film. In this stage the solute
concentration on the surface is expected to become larger than the
concentration in the bulk. Finally, the radial flow comes almost to
a halt and solvent evaporation results in rapid increase of solute
concentration in the film. Several factors might lead to
preferential in-plane alignment of the nanorod solution in this
process. If the nanorods have a strong tendency to segregate to the
surface of the liquid film driven by their low surface tension in
the case of OCTA-ZnO it is possible that the critical concentration
for forming a lyotropic phase on the surface is reached at a stage
when there is still a sufficiently high, in-plane radial flow
velocity to align the nanorods in the plane (scheme 9(c)). Ideally,
in this situation one might expect a uniform radial alignment of
the rods. This is clearly not observed (FIG. 7), however, it is
possible that such radial flow alignment is not preserved when the
liquid crystalline phase solidifies and crystalline domains with
random orientation of the nanorods in the plane nucleate. A second
reason for the observed in-plane alignment of the rods in
spin-coated films could be that the lyotropic phase forms at a
point at which the film has already thinned down to a thickness
that is comparable or less than the length of the nanorods. In this
case capillary forces might cause the nanorods to prefer to align
in the plane of the substrate. Generally, the alignment process in
spin-coated films is complex. We have found for example, that in
more concentrated solutions (50 mg/ml) processed by spin-coating
the nanorods are not aligned in the plane, but adopt a tilted
orientation with respect to the substrate. This might be an
indication that in concentrated solutions nucleation occurs at a
stage when the liquid film thickness is still significantly larger
than the length of the nanorods.
[0118] This establishes a number of selection criteria for the
ligand of the nanorods. The ligand is preferably selected to lower
the surface tension of the nanoparticles. This can be achieved in a
number of ways such as using a long alky chain ligand, use of a
branched ligand or by incorporating specific chemical groups such
as fluorinated groups into the ligand. Preferably the ligand acts
as a surfactant in the solvent which is used for deposition of the
nanoparticles. This means that when the ligand molecule is
dissolved in the solvent it lowers the surface tension of the
solvent, i.e., by measuring and comparing the surface tension (for
example, by using a Wilhelmy balance) of the solvent with and
without ligand added suitable ligands that have a tendency to
promote segregation of the nanoparticles to the surface can be
selected. Ligands are preferred for which the surface tension of
the solution with a certain concentration of ligand molecules is
lower than that of the pure solvent.
[0119] Similarly, suitable ligands can also be selected by
measuring and comparing the surface tension of the solvent with and
without ligand modified nanoparticles. Ligands are preferred for
which the surface tension of the nanoparticle loaded solution is
lower than that of the pure solvent.
[0120] The ligand is also selected to have a low boiling point and
relatively weak chemical bond with the surface in order to enable
removal of the ligand from the surface by low-temperature
annealing, typically at or below 250.degree. C. The ligand is
selected to interact favorably with the solvent used for the
deposition of the nanoparticles in order to enable a high loading
of nanoparticles and a stable dispersion. Finally, the length of
the ligand is selected to provide an acceptable compromise between
the required low surface tension of the nanoparticles and the need
for a dense packing of the as-deposited nanoparticles. In the case
of alkylamine ligands the use of octylamine provides such a
compromise.
EXAMPLE 7
TFT Performance as a Function of Ligand Length
[0121] The ability to self-assemble the nanorods into well-aligned
structures can be exploited to achieve significantly improved
electrical transport in solution-processed ZnO films. We can
control the alignment of the nanorods in spin-coated films that can
easily be integrated into FET structures, and using ligands such as
OCTA-ZnO it is possible to produce films with favourable in-plane
alignment of the nanorods and domain size of several .mu.m's. Here
we investigate the correlation between the field-effect transistor
performance and the nanorod alignment in spin-coated ZnO films with
different ligands.
[0122] FETs were fabricated in a standard bottom-gate, top-contact
configuration using highly doped silicon wafers acting as gate
electrodes with a 300 nm SiO.sub.2 gate dielectric (FIG. 10(a)).
The SiO.sub.2 surface was modified by a self-assembled monolayer of
hexamethyldisilazane (HMDS) prior to spin-coating of the active ZnO
nanorod film. The HMDS substrate modification was found to lead to
better device performance compared to films deposited onto
unmodified, hydrophilic SiO.sub.2. HMDS and other self-assembled
monolayers render the surface hydrophobic, and are widely used in
organic TFTs. We did not observe significant differences in the
film morphology between films on HMDS and untreated SiO.sub.2
substrates, the improvement in device performance might be related
to a decrease in trap states at the ZnO/SiO.sub.2 interface. After
spin-coating the films were annealed at 230.degree. C. in forming
gas atmosphere to remove the ligand. The device was completed by
evaporating Al top source-drain contacts through a shadow mask,
which we found led to better device performance than gold contacts.
Al has a work function that is matched well to the
electron-affinity of ZnO and should exhibit only a small barrier
for electron injection from Al into the conduction band of ZnO. The
device channel length L and width W are L=90 .mu.M and W=3 mm.
[0123] The transfer characteristics of the device fabricated from
nanorods with different ligands are plotted in FIG. 10(a). All the
devices fabricated from ZnO nanorods with different ligands show
n-type behavior. The saturated mobility (.mu..sub.sat) is
calculated by fitting a straight line to the plot of the square
root of I.sub.d vs. V.sub.g, according to the expression (for the
TFT saturated region): I.sub.d=(C.sub.i
.mu..sub.satW/2L)(V.sub.g-V.sub.TH).sup.2 for the case of
V.sub.d<V.sub.g-V.sub.TH, where C.sub.i=11.4 nF/cm.sup.2 is the
capacitance of the 300 nm SiO.sub.2 gate dielectric. V.sub.TH is
the threshold voltage which is typically less than 20V. The TFT
devices with OCTA-ZnO as active layer exhibit significantly higher
performance than the films with lower ligand length. OCTA-ZnO films
have saturated mobility of 0.1.about.0.12 cm.sup.2V.sup.-1s.sup.-1
and an ON/OFF ratio of 10.sup.5.about.10.sup.6. This is also
evident from the output current-voltage characteristics of the
device from OCTA-ZnO (FIG. 10(b)). The mobility extracted from the
linear region (V.sub.d=5V) has almost the same values as
.mu..sub.sat suggesting a low contact resistance. The mobility
value is 5-6 times higher than that of BUTA-ZnO TFT with saturated
mobility of 0.015-0.02 cm.sup.2V.sup.-1s.sup.-1. Device with
HEXA-ZnO show intermediate performance. This improvement of
mobility with ligand length is fully consistent with the
morphological results described earlier, and shows that a high
degree of in-plane nanorod alignment and a large domain size is
beneficial to transport in the nanorod films. This result also
provides evidence that transport in the ZnO films is not limited by
residual ligands remaining on the surface of the nanorods after the
annealing step since in this case one would expect the films with
longer ligands to exhibit poorer performance.
[0124] We have also investigated whether the device performance can
be enhanced further by subjecting the nanorod films to a
post-deposition hydrothermal growth step in aqueous zinc solution.
As shown in Example 3 this leads to growth of the nanorods on the
substrate, fusing of the rods and results in further device
improvement. Initial experiments have shown that after the
hydrothermal growth step mobility values of 1.2-1.4
cm.sup.2V.sup.-1s.sup.-1 and ON/OFF ratio of
10.sup.6.about.10.sup.7 can be achieved in highly crystalline films
of OCTA-ZnO.
[0125] This self-assembly methods presented here are simple, and
they can be applied to solution deposition techniques which are
capable of producing uniform films over large areas. Examples are
techniques, such as, but not limited to, spin coating, blade
coating, curtain coating, gravure coating, capillary coating, zone
casting. Particularly preferred are techniques that induce a
unaxial, in-plane liquid flow, which can be used to align the
low-surface tension ligand-modified nanoparticles uniaxially in the
plane along the direction of flow.
[0126] Alternatively, printing techniques can be used for the
deposition of the low-surface tension ligand-modified
nanoparticles, such as, but not limited to, inkjet printing, screen
printing, offset printing, gravure printing, or flexographic
printing.
[0127] Colloidal nanorods can thus be used for solution-processed
electronic devices that can achieve performance levels which are
not accessible with solution-processed organic semiconductors. The
techniques for nanorod self-assembly can also be applied to other
devices, including optoelectronic devices. The self-assembly
techniques described in the present invention can also be applied
to device configurations which require an out-of-plane alignment of
the nanorods, such as solar cells or light-emitting diodes.
[0128] The present invention can be applied to other inorganic
semiconducting nanorod materials, including, but not limited to,
nanorods based on, comprising or consisting of SnO.sub.2, Si, Ge,
SiGe, GaAs, GaP, InP, InAs, In.sub.2O.sub.3, ZnO, ZnS, ZnSe, ZnTe,
CdS, CdSe, CdTe, HgS, HgSe, HgTe, PbS, PbSe, PbTe, SnO.sub.2,
SnS.sub.2, SnSe.sub.2, SnTe.sub.2.
[0129] The processes and techniques described herein apply to the
fabrication of a semiconducting layer and may be used in the
manufacture of any devices that comprise a semiconducting layer,
such as diodes, FETs, MOSFETs, BJTs and optoelectronic devices.
[0130] The present invention can also be applied to more complex
shaped elongate nanoparticles that can be dispersed in solution,
such as platelets, tetrapods, and other, branched
nanoparticles.[43] Such particles could be substituted for some or
all of the nanorods disclosed in the above examples.
[0131] The post-deposition growth of the nanorods on the substrate,
that fuses the individual, as-deposited nanorods can be performed
by other solution- or vapour-phase growth techniques, such as, but
not limited to, liquid phase epitaxy or chemical vapour
deposition.
[0132] The present invention is not limited to the foregoing
examples. Aspects of the present invention include all novel and/or
inventive aspects of the concepts described herein and all novel
and/or inventive combinations of the features described herein.
[0133] The applicant hereby discloses in isolation each individual
feature described herein and any combination of two or more such
features, to the extent that such features or combinations are
capable of being carried out based on the present specification as
a whole in the light of the common general knowledge of a person
skilled in the art, irrespective of whether such features or
combinations of features solve any problems disclosed herein, and
without limitation to the scope of the claims. The applicant
indicates that aspects of the present invention may consist of any
such individual feature or combination of features. In view of the
foregoing description it will be evident to a person skilled in the
art that various modifications may be made within the scope of the
invention.
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