U.S. patent application number 11/911565 was filed with the patent office on 2008-11-13 for method of preparing zinc oxide nanorods on a substrate by chemical spray pyrolysis.
This patent application is currently assigned to TALLINN UNIVERSITY OF TECHNOLOGY. Invention is credited to Tatjana Dedova, Malle Krunks, Ilona Oja-Acik.
Application Number | 20080280058 11/911565 |
Document ID | / |
Family ID | 36649423 |
Filed Date | 2008-11-13 |
United States Patent
Application |
20080280058 |
Kind Code |
A1 |
Krunks; Malle ; et
al. |
November 13, 2008 |
Method of Preparing Zinc Oxide Nanorods on a Substrate By Chemical
Spray Pyrolysis
Abstract
A method of preparing nanostructured zinc oxide layers on a
substrate by chemical spray pyrolysis at moderate deposition
temperatures from 350.degree. C. to 600.degree. C. is disclosed. An
aqueous or aqueous alcoholic solution comprising zinc chloride or
zinc acetate as precursors is prepared and sprayed onto the
preheated substrate so that the precursor reacts to form zinc oxide
layer on the substrate. Thiourea or urea may be also added to the
solution. Glass, silicon, or metal oxide covered glass can be used
as the substrate.
Inventors: |
Krunks; Malle; (Tallinn,
EE) ; Oja-Acik; Ilona; (Tallinn, EE) ; Dedova;
Tatjana; (Tallinn, EE) |
Correspondence
Address: |
Vern Maine & Associates
100 MAIN STREET, P O BOX 3445
NASHUA
NH
03061-3445
US
|
Assignee: |
TALLINN UNIVERSITY OF
TECHNOLOGY
Tallinn
EE
|
Family ID: |
36649423 |
Appl. No.: |
11/911565 |
Filed: |
April 13, 2006 |
PCT Filed: |
April 13, 2006 |
PCT NO: |
PCT/EE2006/000002 |
371 Date: |
June 9, 2008 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60671232 |
Apr 14, 2005 |
|
|
|
Current U.S.
Class: |
427/453 ;
257/E21.464 |
Current CPC
Class: |
H01L 21/02554 20130101;
C23C 4/11 20160101; C23C 18/1245 20130101; C23C 18/1258 20130101;
B05D 1/02 20130101; H01L 21/02603 20130101; C23C 18/1291 20130101;
H01L 21/0237 20130101; C23C 18/1216 20130101; H01L 21/02628
20130101 |
Class at
Publication: |
427/453 |
International
Class: |
C23C 4/10 20060101
C23C004/10 |
Claims
1-25. (canceled)
26: Method of preparing nanostructured zinc oxide (ZnO) layers on a
substrate, having a first surface and a second surface, the method
comprising: heating the substrate to a predetermined temperature;
atomizing a solution, comprising a precursor, an additive, and a
solvent, into small discrete droplets using spray technique,
whereas said precursor is selected from the group of zinc chloride
(ZnCl.sub.2) and zinc acetate (Zn(CH.sub.3COO).sub.2) and said
additive is selected from the group of thiourea
(SC(NH.sub.2).sub.2) and urea (OC(NH2)2); and depositing the
atomized solution on the first surface of the substrate, using
predetermined solution feeding rate, whereas the solvent evaporates
when reaching the substrate, and said precursor reacts to form the
nanostructured zinc oxide layer.
27: The method according to claim 26, wherein a precursor molar
ratio ZnCl.sub.2:SC(NH.sub.2).sub.2 in the solution is from about
1:1 to about 4:1.
28: The method according to claim 26, wherein a precursor molar
ratio ZnCl.sub.2:OC(NH.sub.2).sub.2 in the solution is from about
1:1 to about 4:1.
29: The method according to claim 26, wherein said solvent is
selected from the group consisting of H.sub.2O, alcohols, and
combinations thereof.
30: The method according to claim 29, wherein said solvent
comprises H.sub.2O and an alcohol, and a ratio of H.sub.2O and said
alcohol is from about 1:1 to about 2:3 by volume.
31: The method according to claim 30, wherein said alcohol is
selected from the group consisting of propanol, isopropanol,
ethanol, methanol, and combinations thereof.
32: The method according to claim 26, wherein the concentration of
zinc chloride is from about 0.01 moles per litre to about 0.4 moles
per litre.
33: The method according to claim 26, wherein the predetermined
solution feeding rate is from about 1 ml/min to about 5 ml/min.
34: The method according to claim 26, wherein the substrate is
selected from a group consisting of glass, a metal oxide covered
glass, Silicon and quartz.
35: The method according to claim 33, wherein the metal oxide is
selected from a group consisting of indium tin oxide, tin oxide,
titanium oxide, and zinc oxide.
36: The method according to claim 26, wherein the depositing is
performed in an open system and air or compressed air is used as a
carrier gas.
37: The method according to claim 35, wherein a flow rate of said
carrier gas is between about 5 l/min to about 9 l/min.
38: The method according to claim 26, wherein nitrogen or argon is
used as a carrier gas.
39: The method according to claim 26, wherein the heating of said
substrate is performed by bringing a heating element into contact
with the second surface of the substrate, and the temperature of
the first surface of the substrate is controlled indirectly by
controlling the temperature of the heating element.
40: The method according to claim 39, wherein soldered metal bath
is used as said heating element, glass with a thickness of about 1
mm is used as said substrate, and a temperature of said soldered
metal in said bath is from about 400.degree. C. to about
650.degree. C.
41: The method according to claim 26, wherein said predetermined
temperature of said first surface of said substrate is from about
350.degree. C. to about 600.degree. C.
42: The method according to claim 27, wherein the predetermined
temperature of said first surface of said substrate is from about
400.degree. C. to about 600.degree. C.
43: The method according to claim 27, wherein the predetermined
temperature of said first surface of said substrate is from about
400.degree. C. to about 600.degree. C.
44: The method according to claim 26, wherein said solvent is
H.sub.2O and wherein the concentration of zinc acetate is from
about 0.1 moles per litre to about 0.4 moles per litre.
45: The method according to claim 26, wherein said substrate is
selected from the group consisting of Silicon and quartz and said
predetermined temperature of said first surface of said substrate
is from about 350.degree. C. to about 650.degree. C.
Description
[0001] This application claims the priority of U.S. provisional
application No. 60/671,232, filed on 14 Apr. 2005, the disclosure
of which is incorporated herein by reference in its entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Technical Field
[0003] The invention relates to zinc oxide (ZnO) nanostructures,
such as nanorods and nanoneedles, and to a method for manufacturing
thereof, and more particularly, to a method of preparing highly
structured zinc oxide layers comprising zinc oxide nanorods or
nanoneedles, on various substrates by chemical spray pyrolysis
(CSP) at moderate deposition temperatures of the substrate (from
about 400 to 600.degree. C.).
[0004] Such nanorods are individual single crystals with high
purity. CSP is technologically simple deposition technique where no
costly equipment is needed. Therefore, the invention provides very
cheap and simple method, compared to alternative methods, for
manufacturing zinc oxide nanostructures.
[0005] Zinc oxide is one of the most promising materials for
optoelectronic applications due to its wide band gap of 3.37 eV and
large exiton binding energy of 60 meV. Zinc oxide nanostructures
have wide range of potential applications also in areas such as
solar cells, field emission devices, chemical and biological
sensors, photocatalysts, light emitting devices, including light
emitting diodes, and nano-sized lasers.
[0006] 2. Background Art
[0007] Flat zinc oxide layers (i.e., as opposed to a layer,
comprising nanorods, nanoneedles, nanowires, etc structures) are
widely used for electronic and optoelectronic devices, for example,
as transparent electrodes in thin film solar cells where
simultaneously a high transparency and a low resistivity is
required, but also in thin film gas sensors, varistors, and surface
acoustic-wave devices.
[0008] Flat zinc oxide layers are conventionally prepared by
several technologies, including sputtering, chemical vapour
deposition, sol-gel deposition, atom layer deposition, molecular
beam epitaxy, and different spray pyrolysis technologies
(ultrasonic spray, pneumatic spray, pressure spray). In contrast to
the other deposition techniques, the advantage of spray technique
is its extreme simplicity. So the capital cost and the production
cost of high quality metal oxide semiconductor films are expected
to be the lowest compared to all other techniques. Furthermore,
this technique is also well suited for mass production systems.
[0009] Chemical spray pyrolysis is a well-known, cheap and simple
deposition technique to prepare thin films of metal oxides,
sulfides and tellurides, etc. for application in electronics and
optoelectronics. U.S. Pat. No. 3,148,084 to Hill (Sep. 8, 1964) for
a process for making conductive film describes a process of making
homogeneous microcrystalline semiconductive and photoconductive
films, e.g. cadmium sulphide. The method was simpler to operate,
and more efficient, versatile and economical than previously known
methods of forming semiconductive layers.
[0010] Spray technologies have been used for different materials
and applications by Chamberlin R. R. et al (Chemical Spray
Deposition for Inorganic films, J. Electrochemical Soc. 113 (1966)
86-89), Feigelson R. S. et al. (II-VI Solid Solution Films By spray
Pyrolysis, J. Appl. Phys. 48 (1977) 3162-3164), Aranovich J. et al
(Optical and Electrical Properties of ZnO Films Prepared by Spray
Pyrolysis for Solar Cell Application, J. Vac. Sci. Technol. 16
(1979) 994-1003), Turcotte R. L. (U.S. Pat. No. 4,338,362 for
Method to synthesize and Produce Thin Films by Spray Pyrolysis,
issued Jul. 6, 1982), Major S. et al (Thin Solid Films, 108 (1983)
333-340, Thin Solid Films, 122 (1984) 31-43, Thin Solid Films, 125
(1985) 179-185), Ortiz S. et al (J. of Non-Crystalline Solids, 103
(1988) 9-13, Materials Chemistry and Physics, 24 (1990) 383-388),
Caillaud F. et al (J. European Ceramic Society, 6 (1990)
313-316).
[0011] To prepare flat films of zinc oxide by spray usually zinc
salts e.g. zinc acetate, zinc nitrate etc. can be used as precursor
materials. Appropriate additives as salts of Indium, Aluminum or
Terbium were added into the spray solution to make the films
electrically conductive (European Patent application No 336574 to
Sener for producing a layer of transparent conductive zinc oxide,
priority date 6 Apr. 1988) and cobaltous or chromium
acetylacetonates to accelerate the growth of the films in spray
process (European Patent No 490493 to Platts for A process for
depositing a layer of zinc oxide onto a substrate, date of filing
14.11.91, priority 12.12.90; U.S. Pat. No. 5,180,686 to Banerjee
for Method for continuously depositing a transparent oxide material
by chemical pyrolysis, issue date Jan. 19, 1993).
[0012] Zinc oxide nanopowder is also widely used, e.g., in
sunscreens, paints, plastics, cosmetics because of its property to
absorb ultra-violet radiation. Different methods are used to
produce such powder. Spherical ZnO microcrystals could be obtained
by spray pyrolysis (see, e.g., M. Andres-Verges, et al, J.
Materials Science 27 (1992) 3756-3762, Kikuo Okuyama et al Chemical
Engineering Science 58 (2003) 537-547, Kang, Y. C. et al Journal of
Aerosol Science, 26 (1995) 1131-1138). In U.S. Pat. No. 6,036,774
to Lieber (filing date 22 Jan. 1997, issue date 14 Mar. 2000) for
method of producing metal oxide nanorods describes metal oxide
nanorods with diameter between 1 and 200 nm and aspect rations
between 5 and 2000, produced by controlled vapour-solid growth
processes in a furnace from a metal vapour source such as a mixture
of a bulk metal oxide powder and carbon powder, and a low
concentration of oxygen gas.
[0013] Rod-like zinc oxide nanoparticles/crystals of different size
are made by deposition from solutions (M. Andres-Verges, et al, J.
Materials Science 27 (1992) 3756-3762), by hydrothermal synthesis
in solutions (Wei H. et al Materials Science and Engineering A, 393
(2005) 80-82, Bai F. et al Materials Letters 59 (2005) 1687-1690,
Guo M. et al Applied Surface Science, In Press, Corrected Proof,
Available online 7 Jan. 2005, Kiwamu Sue et al Materials Letters,
58 (2004) 3350-3352), by chemical bath deposition (A. M. Peiro et
al Thin Solid Films, In Press, Corrected Proof, Available online 20
Jan. 2005, Zhuo Wang Journal of Solid State Chemistry, 177 (2004)
2144-2149, etc.), thermal or physical vapour deposition
(Mardilovich P. et al U.S. Pat. No. 6,770,353 B1; D. W. Zeng et al,
Journal of Crystal Growth, 266 (2004) 511-518), chemical vapour
deposition (G. Z. Wang et al. Materials Letters, 58 (2004)
2195-2198, Jae Young Park et al, Journal of Crystal Growth, In
Press, Corrected Proof, Available online 15 Dec. 2004, US Patent
Applications No 2003/0213428A1 to X. Lu et al, Nos US2004/0127130A1
and 2004/0252737A1, and PCT application WO 2004/114422A1 to Yi G.
C. et al).
[0014] However, the background art does not suggest that chemical
spray pyrolysis can be used for preparing highly structured zinc
oxide, namely nanostructured layers comprising ZnO nanorods or
nanoneedles, on various substrates.
DISCLOSURE OF THE INVENTION
[0015] The method of growing nanostructured zinc oxide (ZnO) layers
on a substrate according to present invention comprises the steps
of heating a substrate to a predetermined temperature, atomizing a
solution, comprising a precursor, such as zinc chloride
(ZnCl.sub.2) or zinc acetate (Zn(CH.sub.3COO).sub.2), and a
solvent, into small discrete droplets using spray pyrolysis; and
depositing the atomized solution to the substrate, using
predetermined solution feeding rate. The solvent evaporates when
the droplets reach the substrate and the precursor reacts to form a
plurality of zinc oxide nanorods (or, in some cases, nanoneedles)
on said substrate.
[0016] Aqueous or aqueous-alcoholic solution of zinc chloride or
zinc acetate is used. Fine droplets of said solution are produced
by atomizing of the solution with the help of ultrazonic or
pneumatic spray techniques. The deposition process is carried out
in air, compressed air, nitrogen or argon are used as carrier
gases.
[0017] The aqueous or aqueous-alcoholic solution of zinc chloride
may additionally contain thiourea (thiocarbamide
SC(NH.sub.2).sub.2) or urea (carbamide, OCN.sub.2H.sub.4). Adding
thiourea or urea to the aqueous or aqueous-alcoholic solution of
zinc acetate may also be useful in some cases.
[0018] The substrate can be, e.g., glass, silicon or quartz (quartz
slide). The substrate can be covered by a flat layer of different
metal oxides, e.g., indium tin oxide, tin oxide, titanium oxide,
zinc oxide.
[0019] The nanocolumnar zinc oxide layers are consisting of
well-developed hexagonal nanorods of single crystal zinc oxide with
length from 50 nm up to six-seven microns, the diameter of rods
could be varied from some tens of nanometers up to 1 micron.
[0020] The shape and size of zinc oxide crystals are controlled by
several parameters, including the growth temperature, stock
solution composition, concentration of precursors in stock
solution, solution feeding rate, type of substrate, type of a flat
layer of metal oxide (also called underlayer), and carrier gas flow
rate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a SEM cross-section of the nanostructured zinc
oxide layer that is deposited from aqueous solution of zinc
chloride (0.05 mol/l) onto glass substrate that was placed onto the
soldered tin bath heated up to 600.degree. C., and using the
solution feeding rate of 2.4 ml/min;
[0022] FIG. 2 is a SEM cross-section of the nanostructured zinc
oxide layer that is deposited from aqueous solution of zinc
chloride (0.1 mol/l) onto glass substrate covered with conductive
indium tin oxide (ITO) layer, whereas the glass substrate was
placed onto the soldered tin bath heated up to 600.degree. C., and
using the solution feeding rate of 2.4 ml/min;
[0023] FIG. 3 is a SEM micrograph of the surface of the
nanostructured zinc oxide layer that is deposited from aqueous
solution of zinc chloride (0.1 mol/l) onto glass substrate covered
with dense film of ZnO:In with thickness of about 300 .mu.m,
whereas the glass substrate was placed onto the soldered tin bath
heated up to 600.degree. C., and using the solution feed rate of
2.4 ml/min;
[0024] FIG. 4 is a SEM cross-section of the nanostructured zinc
oxide layer that is prepared from zinc chloride solution with
concentration of 0.2 mol/l onto the glass substrate that was placed
onto the soldered tin bath heated up to 600.degree. C., and using
the solution feed rate of 1.7 ml/min;
[0025] FIG. 5 is a SEM cross-section of the nanostructured zinc
oxide layer that is prepared from zinc chloride solution with
concentration of 0.2 mol/l onto glass substrate that was placed
onto the soldered tin bath heated up to 600.degree. C., and using
the solution feed rate of 3.3 ml/min;
[0026] FIG. 6A is a SEM micrograph of the nanostructured zinc oxide
layer that is prepared from zinc chloride solution with
concentration of 0.1 mol/l onto glass substrate that was placed
onto the soldered tin bath heated up to 525.degree. C., and using
the solution feed rate of 2.3 ml/min;
[0027] FIG. 6B is a SEM cross-section of the nanostructured zinc
oxide layer that is prepared from zinc chloride solution with
concentration of 0.1 mol/l onto glass substrate that was placed
onto the soldered tin bath heated up to 525.degree. C., and using
the solution feed rate of 2.3 ml/min;
[0028] FIG. 7 is a SEM cross-sectional image of the nanostructured
zinc oxide layer that is deposited from the aqueous solution
containing zinc chloride (0.05 mol/l) and thiourea (tu) at molar
ratio of Zn:S=1:1 onto glass substrates that was placed onto the
soldered tin bath heated up to 620.degree. C.;
[0029] FIG. 8 is a SEM cross-sectional image of the nanostructured
zinc oxide layer that is deposited from the aqueous solution
containing zinc chloride (0.05 mol/l) and thiourea at molar ratio
of Zn:S=3:1, deposited onto glass substrates that was placed onto
the soldered tin bath heated up to 620.degree. C.;
[0030] FIG. 9 is a SEM cross-sectional image of the nanostructured
zinc oxide layer that is deposited from the isopropanol and water
solution (in ration 1:1 by volume) with zinc chloride concentration
of 0.1 mol/l, deposited onto glass substrate that was placed onto
the soldered tin bath heated up to 525.degree. C., and solution
feed rate 2.0 ml/min;
[0031] FIG. 10 is a SEM cross-sectional image of the nanostructured
zinc oxide layer that is deposited from the solution containing
zinc chloride (0.1 mol/l) and urea at molar ratio of 1:1 onto glass
substrates that was placed onto the soldered tin bath heated up to
580.degree. C., and solution feed rate 2.2 ml/min;
[0032] FIG. 11 is a ratio of zinc oxide (002) peak intensity to
(101) plane intensity (I(002)/I(101)) in the XRD pattern for the
layers with different amount of thiourea (tu) in the stock solution
of the samples (prepared at constant tin bath temperature of
620.degree. C., temperature at the substrate surface (the growth
temperature) approximately 500.degree. C.);
[0033] FIG. 12 is an XRD pattern of the sample that is depicted on
FIG. 1;
[0034] FIG. 13 is an XRD pattern of the sample that is depicted on
FIG. 2;
[0035] FIG. 14 is an XRD pattern of the sample, depicted on FIG.
3;
[0036] FIG. 15 is a RHEED pattern of a zinc oxide nanorod;
[0037] FIG. 16 is a near band edge PL spectrum of zinc oxide
nanorods;
[0038] FIG. 17 is a SEM cross-sectional image of the nanostructured
zinc oxide layer that is deposited from aqueous-alcoholic solution
of zinc acetate (0.2 mol/l) onto glass substrate that was placed
onto soldered tin bath heated up to 450.degree. C.;
[0039] FIG. 18 is a SEM micrograph of the surface of the
nanostructured zinc oxide layer that is deposited from
aqueous-alcoholic solution of zinc acetate (0.2 mol/l) onto glass
substrate that was placed onto soldered tin bath heated up to
450.degree. C.;
[0040] FIG. 19 is a near band edge PL spectrum of nanostructured
zinc oxide layer comprising zinc oxide nanoneedles.
MODES FOR CARRYING OUT THE INVENTION
[0041] The process of preparing nanostructured zinc oxide layers
comprising nanorods or nanoneedles on a substrate according to
present invention requires a solution comprising a precursor, such
as zinc salt, e.g., zinc chloride (ZnCl.sub.2), or zinc acetate
(Zn(CH.sub.3COO).sub.2). Aqueous or aqueous-alcoholic solution can
be used, whereas the concentration of zinc chloride in the solution
can be from about 10 mmol up to about 0.4 mol per liter, and
preferably from about 0.05 mol/l to 0.2 mol/l.
[0042] Suitable substrate for the nanostructured zinc oxide layer
is glass, silicon, quartz, or metal oxide (such as indium tin
oxide, titanium oxide, zinc oxide) covered glass. The substrate
must be heated up, whereas the temperature of the surface (on which
the nanostructured ZnO layer is to be prepared--hereinafter also
called the first surface), prior to deposition is from about 400 to
about 650.degree. C. for Silicon and quartz and 400.degree. C. to
600.degree. C. for glass and metal oxide covered glass. This
temperature is also known as growth temperature.
[0043] Different methods can be used for heating the substrate. For
example, to guarantee the homogeneous temperature of the substrate,
substrate is placed onto a soldered metal bath (the surface that is
facing the soldered metal is hereinafter also called the second
surface), and the temperature of the first surface of the substrate
is controlled indirectly by controlling the temperature of the
soldered metal. The metal having low vapor pressure, e.g., tin (Sn)
could be used as the soldered metal.
[0044] Also, heat plate can be used as heating element instead of
soldered metal bath.
[0045] It is apparent that a temperature difference exists between
the temperature of the heating element (e.g., soldered metal) and
the temperature of the first surface of the substrate, whereas this
difference is substantial for substrates like glass and metal oxide
covered class and nearly zero for Silicon. For example, if soldered
metal bath is used, the temperature of the soldered metal is about
70 to about 130 degrees higher than the growth temperature for the
range of growth temperatures between about 400.degree. C. to
600.degree. C. for a glass/quartz substrate with a thickness of
about 1 mm.
[0046] Other methods, known in the art, can be used to heat the
substrate.
[0047] Lower growth temperatures are preferred as less energy is
needed for preheating the substrate and for maintaining the
predetermined temperature.
[0048] Atomization, i.e. producing a spray of small droplets of the
solution of a required size, is then carried out. Any suitable
means can be used, e.g., ultrasonic spray atomizer, pneumatic spray
atomizer.
[0049] The spray of small droplets of the solution is then directed
to the substrate, thereby creating a layer of nanostructured zinc
oxide, comprising nanorods or nanoneedles, on the substrate. The
orientation of the nanorods or needles does not depend on the
direction of the spray stream is applied on the substrate, but
rather on the properties of substrate (or the layer of metal oxide
on the substrate, as the case might be).
[0050] The deposition can be carried out in an open system.
Compressed air (at 2-3 bar) can be used as a carrier gas for the
deposition process. However, also nitrogen, or argon can be used,
if needed. A flow rate of the carrier gas is preferably from about
5 to about 9 l/min.
[0051] According to another embodiment of the invention, zinc
chloride is dissolved in a solvent, comprising water and suitable
alcohol, such as propanol, isopropanol, ethanol or methanol, e.g.,
in ratio 1:1 to 2:3 (by volume). Aqueous-alcoholic solution allows
the process to be carried out at the lower temperatures of the
heating element compared to when aqueous solution is used.
[0052] According to another embodiment of the invention, a solution
additionally comprises thiourea. The amount of thiourea is selected
so that the molar ratios of precursors Zn:S is from 1:1 to 4:1.
[0053] Adding thiourea to the solution allows to grow the film
consisting of highly c-axis orientated ZnO columns (FIG. 8)
[0054] According to another embodiment of the invention, a solution
additionally comprises urea (carbamide, OC(NH.sub.2).sub.2) as a
precursor, whereas a precursor ratio ZnCl.sub.2: OC(NH.sub.2).sub.2
in the solution is from about 1:1 to about 4:1.
[0055] According to another embodiment of the invention, zinc
acetate is used as precursor, i.e., zinc acetate dihydrate is
dissolved in aqueous or aqueous-alcoholic solution. Zinc oxide
layers comprising nanoneedles (with shape of cones and size of:
diameter at bottom from 5-10 to 50 nm and length up to 200 nm) in
between and on leaf-like grains/on the surface of ZnO film can be
prepared. The deposition temperature can be varied from about
350-450.degree. C., preferably 370-400.degree. C. Solution
concentrations can be varied from about 0.1 mol/l to about 0.4
mol/l.
EXAMPLES
[0056] Several samples of zinc oxide nanocolumnar layers were
prepared, whereas the following parameters were varied: growth
temperature, stock solution composition, concentration of the
precursors in stock solution, solution feeding rate, type of
substrate, type of underlayers (metal oxides), and carrier gas flow
rate. Samples were studied by the techniques of X-ray diffraction
(XRD), scanning electron microscopy (SEM), transmission electron
microscopy (TEM), and photoluminescence (PL). The results are shown
in FIGS. 1 to 19.
[0057] The solutions were prepared at the room temperature (from
about 18 to about 25.degree. C.), but generally, the temperature of
the solution is not critical.
[0058] Zinc chloride (pro analysis, Merck) or zinc acetate
dihydrate (pro analysis, Merck), thiourea (pro synthesis, Merck),
Urea (pro synthesis, Merck), 2-propanol (pro analysis, Merck),
Ethanol (pro analysis, Merck), deionized water (with specific
resistance 18 M.OMEGA..cm) were used as starting materials.
[0059] A soldered metal bath was used as a heating element. The
bath is a custom-made stainless steel cylinder with diameter 80 mm,
depth 20 mm, compromising a cavity for a thermocouple. Temperature
of the bath was set and electronically controlled using a
thermocouple which is directly contacted with the bath and a
temperature controller (Love 16010 by Dwyer Instruments). Solution
was atomised using air atomizing nozzle (W/O SU 1/4JN-SS by
Spraying Systems; allows to set different solution flow rates),
comprising fluid cap PF1650-SS and air cap PA64-SS. Carrier gas
flow rate was controlled by a flowmeter EK-4AR (Kytolo
Incorporated).
[0060] The layers are consisting of well-developed hexagonal rods
of zinc oxide with length from 500-800 nm up to 7000 nm, the
diameter of rods could be varied from 20 nm up to 1000 nm. The
aspect ratio (length to diameter) of the crystals is from 1.5 up to
20.
Study by X-Ray Diffraction (XRD)
[0061] XRD diffraction patterns were recorded for the prepared
layers deposited onto different substrates. The replicas of
deposited layer on the diffractograms are belonging to the
hexagonal zinc oxide (PDF 36-1451) independent of the substrate at
deposition temperatures 400-600.degree. C. (it should be
appreciated that if the solution contains thiourea, the temperature
will increase as the decomposition of zinc chloride thiourea
complex compound formed in solution is exothermic process (runks M.
et al Journal Thermal analysis and Calorimetry, 72 (2003) 497-506).
The crystallites in the film are orientated in the (002) direction
(c-axis perpendicularly to the substrate) if grown onto the glass
and conductive oxide covered substrates (FIGS. 12 and 13). The
ratio of the peak intensities (I(002)/I(101)) is about 10 when ZnO
nanorods were prepared onto glass or ITO substrates. Depositing the
solution onto thin flat ZnO film, the crystallites in the layer
show preferred orientation in the (101) direction (FIG. 14). Flat
ZnO film has the thickness of 50-200 nm and is prepared by spray
pyrolysis from the solution of zinc acetate dihydrate dissolved in
deionized water. Indium was added in amount of 1 at % (from indium
chloride) into the solution to make flat films conductive. (It is
apparent that Flat ZnO films could be prepared by other methods as
well, for example, by RF magnetron sputtering technique). Appears
that using thiourea in solution allows to grow highly c-axis
orientated rods/crystals of ZnO, the evolution of the preferred
orientation by the Zn:S molar ratio in solution is presented in
FIG. 11.
Study by Transmission Electron Microscope (TEM)
[0062] The structure of sprayed nanorods was studied on a TEM
EMV-100BR. Both, bright field (B.F.) and dark field (D.F.) images
were studied. TEM and reflective high energy electron diffraction
(RHEED) investigations were carried out at 100 kV accelerating
voltage. A standard C(Pt) replicas method was used. The RHEED
pattern of the nanorod is presented in FIG. 15. TEM study confirms
that grown rods are single crystals of ZnO.
Photoluminescence (PL) Study
[0063] The near band edge photoluminescence (PL) spectrum of zinc
oxide nanorods measured at 10 K (laser exitation wavelength 325 nm)
is presented in FIG. 16. PL spectrum shows very sharp emission peak
at 3.356 eV, with two sattelites at 3.361 and 3.376 eV. The
recorded near band edge photoluminescence spectrum and absence of
PL green emission band verifies high purity and perfect
crystallinity of zinc oxide nanorods. PL spectrum in UV region of
the sample comprising nanoneedles on the surface is presented in
FIG. 19, showing that the zinc oxide nanoneedles are also of high
purity and with perfect crystallinity.
[0064] The exemplary embodiments presented herein illustrate the
principles of the invention and are not intended to be exhaustive
or to limit the invention to the form disclosed; it is intended
that the scope of the invention be defined by the claims appended
hereto and their equivalents.
* * * * *