U.S. patent application number 12/139207 was filed with the patent office on 2009-01-29 for sensing devices from molecular electronic devices utilizing hexabenzocoronenes.
This patent application is currently assigned to The Trustees Of Columbia University In The City Of New York. Invention is credited to Xuefeng Guo, Philip Kim, Mathew Benjamin Myers, Colin Nuckolls, Shengxiong Xiao.
Application Number | 20090027036 12/139207 |
Document ID | / |
Family ID | 38609996 |
Filed Date | 2009-01-29 |
United States Patent
Application |
20090027036 |
Kind Code |
A1 |
Nuckolls; Colin ; et
al. |
January 29, 2009 |
SENSING DEVICES FROM MOLECULAR ELECTRONIC DEVICES UTILIZING
HEXABENZOCORONENES
Abstract
The present invention generally relates to the fabrication of
molecular electronics devices from molecular wires and Single Wall
Nanotubes (SWNT). In one embodiment, the cutting of a SWNT is
achieved by opening a window of small width by lithography
patterning of a protective layer on top of the SWNT, followed by
applying an oxygen plasma to the exposed SWNT portion. In another
embodiment, the gap of a cut SWNT is reconnected by one or more
difunctional molecules having appropriate lengths reacting to the
functional groups on the cut SWNT ends to form covalent bonds. In
another embodiment, the gap of a cut SWNT gap is filled with a
self-assembled monolayer from derivatives of novel contorted
hexabenzocoranenes. In yet another embodiment, a device based on
molecular wire reconnecting a cut SWNT is used as a sensor to
detect a biological binding event.
Inventors: |
Nuckolls; Colin; (New York,
NY) ; Guo; Xuefeng; (New York, NY) ; Kim;
Philip; (New York, NY) ; Xiao; Shengxiong;
(New York, NY) ; Myers; Mathew Benjamin; (Apoche,
OK) |
Correspondence
Address: |
BAKER BOTTS L.L.P.
30 ROCKEFELLER PLAZA, 44TH FLOOR
NEW YORK
NY
10112-4498
US
|
Assignee: |
The Trustees Of Columbia University
In The City Of New York
New York
NY
|
Family ID: |
38609996 |
Appl. No.: |
12/139207 |
Filed: |
June 13, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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PCT/US2006/061570 |
Dec 4, 2006 |
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12139207 |
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60750994 |
Dec 15, 2005 |
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60750993 |
Dec 15, 2005 |
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60762095 |
Jan 25, 2006 |
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60814604 |
Jun 16, 2006 |
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Current U.S.
Class: |
324/76.11 ;
427/77; 977/750 |
Current CPC
Class: |
G01N 33/5438 20130101;
Y10S 977/721 20130101; G01N 27/128 20130101; H01L 51/0083 20130101;
Y10S 977/701 20130101; H01L 51/0591 20130101; H01L 27/285 20130101;
C12Q 1/002 20130101; H01L 51/0076 20130101; G01N 2610/00 20130101;
H01L 51/0056 20130101; B82Y 10/00 20130101; H01L 51/0036 20130101;
H01L 51/0048 20130101 |
Class at
Publication: |
324/76.11 ;
427/77; 977/750 |
International
Class: |
G01N 27/00 20060101
G01N027/00; B05D 5/12 20060101 B05D005/12 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0002] The invention described herein was funded in part by grants
from the National Science Foundation, NSF Award Number CHE-0117752
and by the New York State Office of Science, Technology, and
Academic Research (NYSTAR) and the Department of Energy,
Nanoscience Initiative (NSET#04ER46118). Colin Nuckolls thanks US
National Science Foundation CAREER award (#DMR-02-37860). The
United States Government may have certain rights under the
invention.
Claims
1. A method of fabricating a molecular sensing device, comprising:
(a) forming a film comprising a compound of the formula 1 on a base
layer, wherein at least one R group comprises an acyl halide; and
##STR00001## (b) disposing two or more electrodes on a surface of
the film opposite the base layer and having one or more gaps there
between.
2. The method of claim 1, wherein the molecular wire comprises
self-assembled single layers of compounds of the formula 1.
3. The method of claim 1, wherein R is selected from the group
consisting of H, O(CH.sub.2).sub.nCH.sub.3 acyl halides, amines,
amides, esters and halogens.
4. The method of claim 1, wherein R is selected from the group
consisting of H, O(CH.sub.2).sub.nCH.sub.3 and acyl halides.
5. The method of claim 1, wherein the base layer comprises a primer
layer formed on a substrate.
6. A molecular sensor comprising: (a) a base layer having first and
second sides; (b) a film comprising a compound of the formula 1
disposed on the base layer; (c) a first electrode and a second
electrode disposed on the film, wherein the first electrode and
second electrode are separated by a gap.
7. The sensor of claim 6, wherein the molecular wire comprises
self-assembled single layers of compounds of the formula 1.
8. The sensor of claim 6, wherein R is selected from the group
consisting of H, O(CH.sub.2).sub.nCH.sub.3, acyl halides, amines,
amides, esters and halogens.
9. The sensor of claim 6, wherein R is selected from the group
consisting of H, O(CH.sub.2).sub.nCH.sub.3 and acyl halides.
10. The sensor of claim 6, wherein the base layer comprises a
primer layer formed on a substrate.
11. A method of determining whether a substance contains an
electron acceptor molecule, comprising: (a) forming a film
comprising a compound of the formula 1 on a base layer, wherein at
least one R group comprises an acyl halide; (b) disposing two or
more electrodes on a surface of the film opposite the base layer
and having one or more gaps there between; (c) filling each of the
one or more gaps between the electrodes with a molecular wire; (d)
measuring an electric characteristic of the device; (e) contacting
the device with a substance containing the molecule; and (f) based
on measuring the electronic characteristic of the device after the
contacting, determining whether the substance includes an electron
acceptor molecule.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of priority of
Provisional Application Nos. 60/750,994 and 60/750,993, both filed
on Dec. 15, 2005; Provisional Application No. 60/762,095, filed on
Jan. 25, 2006; and Provisional Application No. 60/814,604, filed on
Jun. 16, 2006.
BACKGROUND OF THE INVENTION
[0003] 1. Technical Field
[0004] The present invention relates to nanotube-based electronic
devices, including devices which incorporate organic molecules.
[0005] 2. Background Art
[0006] The field of molecular electronics has become one of the
most exciting technology areas in recent years. Molecular
electronics devices are significantly smaller, more energy
efficient and less expensive to manufacture than their
silicon-based counterparts. They are regarded as one of the most
promising technological alternatives to overcome the inherent
scaling limits of silicon devices.
[0007] A basis for molecular electronics lies in organic molecules
that are capable of conducting electricity and switching between on
and off states as a result of external manipulations (in a similar
manner as silicon-based transistors). One way to build such a
molecular electronic device is to use an organic film as an active
channel between the metallic source and drain electrodes. The
molecular structure and a molecule's capability of packing in some
form of ordered structure are crucial to facilitate electron
transport through the channel. However, the selection of
appropriate molecules has proven to be a great challenge.
[0008] An alternative way to build such a device is to bridge two
ends of an individual molecule directly to the source and drain
electrodes. This method does not require that the molecule form any
ordered structure, and will result in a circuit with much smaller
channel region with extraordinary properties. However, due to the
constraints of traditional lithography, the gaps between the metal
electrodes are usually large compared to the size of small organic
molecules, making the bridging very difficult.
[0009] One feature of the latter kind of devices is very small
contact areas between the conducting molecules and the electrodes.
As a result, the electron transport at the junction points between
the molecular wires and metal electrodes becomes significant in the
circuit characteristics. However, bonding between organic molecules
and metal electrodes is difficult to accomplish, and is notoriously
ill-defined even when accomplished. For example, as reported in M.
A. Reed et al., Science vol. 278, p. 252 (1997); A. Salomon et al.,
Adv. Mater. vol. 15, p. 1881 (2003), no methods have been
identified to control the type of metal-molecule bonding in the
most well-studied system involving thiolated molecules on Au
contacts. Moreover, as reported in H. Basch et al., Nano Lett. vol.
5, p. 1668 (2005), even if more conductive contact chemistry is
used, such as carbenes on transition metals and on metal carbides,
molecular-scale metal electrodes are extremely difficult to
fabricate and lack specific chemistry for molecular attachment at
their ends. This ill-defined bonding may result in unpredictable
transport properties of electrons through the devices.
[0010] Carbon nanotubes provide new alternatives in molecular
electronics research. Carbon nanotubes are a unique carbon-based
molecular structure, consisting of graphitic layers wrapped to
cylinders, usually having an extremely high length/width ratio.
Carbon nanotubes can have multi-walls on their cylindrical shells,
or only a single atomic layer. The latter is referred to as Single
Wall Carbon Nanotubes ("SWNTs"), which have narrower diameters
(typically in the range of 1.about.2 nm) and fewer defects on their
molecular structure than their multi-walled counterparts. Depending
on their chirality and diameters, SWNTs may be metallic or
semiconducting. Due to their intriguing structure and unique
electronic properties, SWNTs have become one of the most actively
studied nanostructures in the past decade, and molecular electronic
devices such as field effect transistors based on semiconducting
SWNTs have been studied with increasing interest. For example, U.S.
Patent Pub. No. 2004/0144972 to Dai et al., discloses a voltage
controllable nanotube device where a gate electrode is capacitively
coupled to a carbon nanotube via high-K dielectric material.
[0011] The high aspect ratio of SWNTs makes them good candidates
for constructing a molecular electronic device because metallic
electrodes can be placed at a distance by traditional lithography
methods. However, this benefit can also be a barrier for new
generation nanometer-scale transistors. Reducing the width of
active channels in these transistors is still a great
challenge.
[0012] SWNTs have also been reported in sensing applications, such
as high sensitivity gas detectors and glucose sensors. See S.
Chopra et al., App. Phys. Lett. vol. 83, p. 2280 (2003); S. Chopra
et al., App. Phys. Lett., vol. 80, p. 4632 (2002); P. W. Barone et
al., Nat. Mat. vol. 4, p. 86 (2005). Physical affinity or chemical
reactivity of SWNTs toward the molecules to be detected is the
basis for these applications. However, since SWNTs have a large
surface area and multitude of potential reaction centers, the
specificity and sensitivity of the detection are still limited.
Accordingly, a need remains for a technique for fabricating
electronic devices from SWNTs with appropriate organic
molecules.
SUMMARY OF THE INVENTION
[0013] The present invention provides techniques for precisely
and/or functionally cutting single SWNTs, and selecting and/or
synthesizing appropriate molecules as molecular wires to bridge the
gap formed in the cut SWNTs.
[0014] In one embodiment, a transistor device is fabricated by
forming a film of a novel contorted hexabenzocoronene on a base
layer and depositing two or more electrodes on top of the film. The
base layer preferably includes a primer layer formed on a
substrate.
[0015] In another embodiment, a transistor device is fabricated by
depositing two or more electrodes on a base layer and having one or
more gaps between the electrodes; and filling the one or more gaps
between the electrodes with one or more self-assembled single
layers of a novel contorted hexabenzocoronene.
[0016] In another embodiment, a device to detect a target molecule
is fabricated by obtaining a device of the present invention and
attaching a molecule on the molecular bridge to detect the target
molecule.
[0017] In yet another embodiment a molecular electronic device is
fabricated by laying a SWNT on a base layer; depositing two or more
electrodes on the SWNT; using a lithographic process to locally cut
the SWNT between the electrodes to form a gap therein; and bridging
the gap with a molecular wire.
[0018] In furtherance, this invention provides a method of
precisely and/or functionally cutting single SWNTs and methods of
selecting and/or synthesizing appropriate molecules as molecular
wires to bridge the gap formed in the cut SWNTs.
[0019] In one embodiment, the cutting of a SWNT is achieved by
opening a window of small width by lithography patterning of a
protective layer on top of the SWNT, followed by applying an oxygen
plasma to the exposed SWNT portion.
[0020] The gap of a cut SWNT may be reconnected by one or more
difunctional molecules having appropriate lengths reacting to the
functional groups on the cut SWNT ends to form covalent bonds.
[0021] In another embodiment, the gap of a cut SWNT is filled with
a self-assembled monolayer from derivatives of contorted
hexabenzocoranenes.
[0022] In yet another embodiment, a device based on molecular wire
reconnecting a cut SWNT is used as a sensor to detect a biological
binding event.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] The accompanying drawings, which are incorporated and
constitute part of this disclosure, illustrate preferred
embodiments of the invention and serve to explain the principles of
the invention.
[0024] FIG. 1 is an illustration of the synthesis and chemical
structure of a compound of formula 1, in accordance with an
embodiment of the present invention.
[0025] FIG. 2 is a schematic diagram illustrating a transistor
device based on a spin-cast film of a compound of the formula 1 in
accordance with an exemplary embodiment of the present
invention.
[0026] FIG. 3 is a schematic diagram illustrating a transistor
device based on a self-assembled monolayer of a compound of the
formula 1 in accordance with an exemplary embodiment of the present
invention.
[0027] FIG. 4 is an illustration of a method of fabricating a
cut-SWNT based device in accordance with an exemplary embodiment of
the present invention.
[0028] FIG. 5 is an illustration of a method for locally cutting a
SWNT in accordance with an exemplary embodiment of the present
invention.
[0029] FIG. 6 is an illustration of a method for bridging the gap
of a cut SWNT using a molecular bridge, in accordance with an
exemplary embodiment of the present invention.
[0030] FIG. 7 is an illustration of exemplary structures of diamine
compounds 9, 10, 11 and 12, in accordance with an embodiment of the
present invention.
[0031] FIG. 8 is an illustration of a method for bridging the gap
of a cut SWNT using a self-assembled monolayer, in accordance with
an exemplary embodiment of the present invention.
[0032] FIGS. 9A-D is an illustration of methods of fabricating and
using cut SWNT based devices for sensing applications.
[0033] FIG. 10A is a graph of transconductance of an exemplary
device in accordance with an embodiment of the present
invention.
[0034] FIG. 11A is a DSC test result of a compound of the formula
1.
[0035] FIG. 11B is a X-ray diffraction result for a compound of the
formula 1.
[0036] FIG. 11C is a polarized light microscopy for a compound of
formula 1.
[0037] FIG. 11D is an illustration of ordered mesophase structure
of a film formed by a compound of the formula 1.
[0038] FIG. 12A is an exemplary scanning electron micrograph of a
cut SWNT in accordance with an embodiment of the present
invention.
[0039] FIG. 12B is an Atomic Force Microscopy (AFM) micrograph of a
cut SWNT in accordance with an embodiment of the present
invention.
[0040] FIG. 13A is a I-V.sub.G plot for an exemplary device from a
cut metallic SWNT connected with a compound of the formula 1 at
source-drain voltage V.sub.SD=50 mV, in accordance with an
embodiment of the present invention.
[0041] FIG. 13B is a I-V.sub.SD plot for the same device in FIG.
13A with no gate bias, in accordance with an embodiment of the
present invention.
[0042] FIG. 13C is a I-V.sub.G plot at V.sub.SD=50 mV for an
exemplary device from a cut semiconducting SWNT connected with a
compound of the formula 9, in accordance with an embodiment of the
present invention.
[0043] FIG. 13D is a I-V.sub.G plot at V.sub.SD=50 mV for an
exemplary device from a cut semiconducting SWNT connected with
compound 12, in accordance with an embodiment of the present
invention.
[0044] FIG. 13E is a I-V.sub.G plot at V.sub.SD=50 mV for an
exemplary device from a cut metallic SWNT connected with compound
11, in accordance with an embodiment of the present invention.
[0045] FIG. 14A is a plot for transistor output at V.sub.G=0 to -5V
in 1V steps for an exemplary device based on a monolayer of an
exemplary compound of the formula 1, wherein R.sub.1 is
O--C.sub.12H.sub.25 and R.sub.2 is COCl, and a cut SWNT, in
accordance with an embodiment of the present invention.
[0046] FIG. 14B is a plot for transfer characteristics for the same
device in FIG. 14A at V.sub.D=-2V, in accordance with an embodiment
of the present invention.
[0047] FIG. 15A is an illustration of a film of an exemplary
compound of the formula 1 wherein each R group is
O--C.sub.12H.sub.25 covering a cut SWNT gap, in accordance with an
embodiment of the present invention.
[0048] FIG. 15B is a plot for transfer characteristics of an
exemplary device illustrated in FIG. 15A, in accordance with an
embodiment of the present invention.
[0049] FIG. 16A is a plot for transistor output at V.sub.G=0 to -5V
in 1V steps for an exemplary device illustrated in FIG. 3 after
being dipped in a TCNQ solution, according to an embodiment of the
present invention.
[0050] FIG. 16B is a plot for transfer characteristics at
V.sub.D=-2V for the TCNQ contacted device in FIG. 16A.
[0051] FIG. 17 is a plot for drain current of an exemplary device
based on a cut SWNT bridged by compound 12 in response to pH change
of a medium, according to an embodiment of the present
invention.
[0052] FIG. 18A is an illustration of an exemplary circuit based on
a fluorenone bridge and a cut SWNT, according to an embodiment of
the present invention.
[0053] FIG. 18B is an illustration of a fluorenone compound 13
reacted to a functionalized Biotin to form an oxime, in accordance
with an embodiment of the present invention.
[0054] FIG. 18C is a plot for electric characteristics of an
exemplary device after incorporation of the Biotin into the
molecular bridge and after its association with Streptavidin, in
accordance with an embodiment of the present invention.
[0055] FIG. 18D is an AFM micrograph of an exemplary device in the
gap area showing the gold label on the Streptavidin, in accordance
with an embodiment of the present invention.
[0056] FIG. 19A is an illustration of a fluorenone compound 13 with
a bis-alkoxylamine tether that can be used to react with a modified
anti-FLAG antibody.
[0057] FIG. 19B is an illustration of binding and unbinding of the
FLAG peptide sequence.
[0058] FIG. 19C is current-voltage curves showing the change in the
ON-state resistance when the device of FIG. 19B binds and release
the antigen.
[0059] Throughout the drawings, the same reference numerals and
characters, unless otherwise stated, are used to denote like
features, elements, components or portions of the illustrated
embodiments. Moreover, while the present invention will now be
described in detail with reference to the Figs., it is done so in
connection with the illustrative embodiments.
DETAILED DESCRIPTION OF THE INVENTION
[0060] The present invention provides techniques for fabricating
devices by cutting SWNTs and forming transistors by inserting novel
organic molecules and the devices fabricated in accordance with the
methods. The devices may be used as small sized transistors in
electronic paper, RFID tags, backplanes for OLED displays, low
temperature replacement for amorphous silicon, among others. The
present invention also provides techniques for constructing sensors
based on such devices. The sensors may be used for a wide array of
applications, for example, the detection of pH of a medium, a
chemical substance, or a biological event.
[0061] The present invention provides for a method of fabricating
transistor devices by forming a thin layer (or film) from
appropriately selected molecules between metallic electrodes. To
function as a molecular wire, the layer should contain long-range
ordered structures to enable electron transport, e.g., a path of
alternating single and double bonds to form a conjugated or
substantially conjugated structure.
[0062] The general molecular formula of appropriate molecules,
contorted hexabenzocoranenes 1, is shown in FIG. 1. R.sub.1 and
R.sub.2 side groups in compound 1 may be same or different, and may
have structures including oxylated linear alkyl chains or
functional groups that are reactive toward appropriate surfaces to
facilitate surface attachment, such as an acyl halide.
[0063] As used herein, the term "contorted hexabenzocoronene"
refers to a new type of hexabenzocoronene or "HBC" whose aromatic
core is distorted away from planarity by steric congestion in its
proximal carbon atoms.
[0064] The terms "molecular wire" and "molecular bridge" are
synonymous and refer to any molecule, in individual or aggregate
form, that could be used to fill the gaps between two closely
placed electrodes and function as a conducting means to complete an
electric circuit.
[0065] The term "molecule" is well understood by those of skill in
the art and encompasses single chemical molecules and biological
macromolecules.
[0066] Referring to FIG. 2, an exemplary transistor device is
fabricated in accordance with an embodiment of the present
invention as shown. A compound of the formula 1, where each R group
is O(CH.sub.2).sub.nCH.sub.3 can be used to form a film 110. A film
110 is formed as the surface of a primer layer 120, which in turn
is laid on top of substrate 130. Metallic electrodes 140 are then
deposited on top of the film.
[0067] As those skilled in the art will appreciate, any known
techniques for depositing layers 110, 120 may be utilized. The film
110 may include one or more molecular layers of the compound. The
primer layer 120 may be SiO.sub.2, and the substrate 130 may be Si
wafer. The spacing between electrodes is preferably less than 100
nm. Further, in a preferred embodiment, the electrodes are less
than 10 nm wide.
[0068] Referring next to FIG. 3, an alternative transistor device
is fabricated in accordance with the present invention is shown. A
compound of the formula 1, where R.sub.1 may be H and/or
O(CH.sub.2).sub.nCH.sub.3 and where R.sub.2 is an acyl halide can
be used to form self-assembled single-layer molecules 180. Metallic
electrodes 150 are deposited on a primer layer 160, which, in turn,
is laid on top of substrate 170. The gap between the electrodes is
then filled with the self-assembled single-layer of molecules of
the compound (180) which are immobilized to the primer layer by
their surface attaching functional groups. Preferably, there are 10
or less molecules in the single-layer spaced about 1 nm apart, and
the gap between the electrodes is about 5 nm.
[0069] Referring to FIG. 4, a method to create nanometer-scale
molecular electronic devices based on SWNTs and molecular wires is
shown. As shown in FIG. 4, the method includes obtaining a SWNT
210; placing the SWNT on a base layer 220; locally cutting the SWNT
230. Finally, the gap formed in the cut SWNT is filled with one or
more molecules 240.
[0070] The devices fabricated in accordance with this method are
characterized with ultrasmall active channels constituted by
molecular wires, which can be manipulated and/or further
functionalized for a variety of sensing applications, as discussed
below.
[0071] Cutting SWNTs can be accomplished using a localized chemical
etching process. For example, referring to FIG. 5, an etching agent
310 is applied on a slit opened on a protective layer 320 coated on
top of the SWNT 330 (which was laid upon a primer layer 340 and
substrate layer 350), leaving two SWNT ends separated by a small
gap and capped with the functional groups 360 resulting from this
process. In a particular embodiment, the devices were put into
TECHNIQUES Series 800 RIE machine. The nanotubes were then locally
cut through the open window by an oxygen plasma (50 W RF power,
oxygen 250 mTorr, for 10 s). The protective layer may be formed by
a polymer, e.g., polymethyl methacrylate ("PMMA"). The primer layer
may be SiO.sub.2, and the substrate layer may be a Si wafer.
[0072] Bridging the cut SWNT ends may be accomplished in several
ways. For example, molecules having appropriate functional groups
370 may be used to react to the functional groups at the cut SWNT
ends to form a molecular bridge 380, as illustrated in FIG. 6. The
bridging molecules may contain conjugation structure, i.e.,
alternating single and double bonds or a substantially conjugated
structure having small breaks in the alternating single and double
bonds, to facilitate electric current to flow through the circuit,
and may be tailor made to have the appropriate size as well as
functional groups at both ends. In one embodiment, compounds with
at least two functional groups on either side, such as diamines of
the formulas 9, 10, 11 and 12 illustrated in FIG. 7, are used to
bridge a cut SWNT gap cut by an oxygen plasma. The resulting device
including one or more molecular bridges spanning the gap of cut
SWNT, each bridge including one individual diamine molecule
connected to the cut SWNT through robust amide bonds at both
ends.
[0073] An alternative way to bridge the cut SWNT ends is to use a
film or self-assembled monolayer, where the constituent molecules
are not necessarily chemically bonded to the cut SWNTs. The
one-dimension nature of the contact of the molecular wire with
SWNTs allows for various objectives, such as fabrication of high
performance transistor devices and wide array of choices in
controlling such devices by manipulating the monolayer
structure.
[0074] In one embodiment, a transistor device is fabricated using a
novel hexabenzocoronene compound 1 attached through the R-groups to
a primer layer for assembly between the gap of a cut SWNT. FIG. 8
illustrates schematically a monolayer of self-assembled stacks
formed by compound of the formula 1, wherein R.sub.1 is
--O(CH.sub.2).sub.nCH.sub.3 and R.sub.2 is an acyl halide, probed
with SWNT electrodes, wherein the primer layer may be SiO.sub.2 and
the substrate may be Si wafer.
[0075] In another embodiment, a transistor device is fabricated by
dropping a compound of the formula 1 onto the gap of a cut SWNT so
that it forms a film covering the gap but does not span the
metallic electrodes. The compound does not necessarily form
chemical bonds to the primer layer surface and does not need to
contain surface reacting groups.
[0076] The electric characteristics of molecular devices formed by
cut SWNTs and molecular bridge(s) are sensitive to local charge
configuration near the molecular bridge. This high charge
sensitivity can be exploited for detection of changes in the
surrounding environment, such as a pH change in a medium, or the
presence of a substance, such as a chemical compound.
[0077] The conductance of a molecular bridge can be influenced by
pH of a medium to which the molecular bridge is exposed if the
resonance structure of the molecular bridge can be altered by
protonations and deprotonations. In one embodiment, a device based
on a cut SWNT and polyaniline molecular bridge(s) is used to detect
pH change in a medium.
[0078] The exposed active channel of the monolayer device shown in
FIG. 6 can be employed for recognition of certain types of
molecules that have strong affinity with the molecular stacks. In
one embodiment, the device is used to detect an electron acceptor
molecule by measuring the electric characteristic of the device;
contacting the device with a substance containing the molecule and
then measuring the electric characteristics of the device.
[0079] The above method may be used to detect various target
molecules of .pi.-electron acceptors that have strong affinity with
the coronene structure shared by compounds of formula 1. For
example, the target molecules may be electron deficient arenes that
are used in explosives such as TNT.
[0080] Since many biological processes cause changes in the
electrostatic environment of the molecular bridges, the devices
based on cut SWNTs and molecular wires may be used as biosensors to
detect nucleic acid hybridization, protein-protein interactions,
and protein conformational changes with single-molecule sensitivity
and at the single molecule/event level. This new level of
sensitivity has not been previously possible with
fluorescence-based techniques. These devices may have broad
practical application in medical diagnostics (genomics and
proteomics), drug discovery, environmental monitoring, and
elsewhere.
[0081] By way of example, FIG. 9 shows how the devices can be
constructed for these sensing applications. In FIG. 9A, a molecule
410 (DNA, RNA, protein or an organic ligand) is attached on the
molecular bridge to detect a target molecule 420 that can bind to
it by physical association. For an attached DNA or RNA, the target
molecule may be its complementary strand; for an attached protein,
such as an antibody, the target molecule can be another protein or
a smaller compound, such as an antigen. If an antibody capable of
recognizing cancer cell tumor markers is attached, devices
providing a biomedical diagnosis for tumor markers may be created.
In FIG. 9B, possible locations (440) of breakdown of a protein,
i.e., amie bonds or disulfide bridges, 430 catalyzed by an enzyme
are detected. In FIG. 9C, a conformation change of a protein 450
induced by a small molecule or a change in the surrounding medium
is detected.
[0082] A variety of chemical reactions can be employed to
incorporate a second molecule into the molecular bridge for sensing
applications illustrated above. In one embodiment, a fluorenone
compound is used as the molecular bridge, and the fluorenone
compound is further derivatized to incorporate a second molecule
through oximation.
EXAMPLES
[0083] A number of Examples for practicing the present invention
are provided below for illustration purpose only. In doing so,
Applicants do not intend to limit the scope of the invention to the
specific embodiments disclosed herein.
Example 1
Preparation of Contorted Hexabenzocoronenes
[0084] The syntheses for preparing contorted hexabenzocoronenes of
formula 1 are schematically shown in FIG. 1. Ketone 3 was
synthesized according to the well-known procedure disclosed in E.
Clar, Chemische Berichte vol. 82, p. 495 (1949), the contents of
which are incorporated by reference herein.
[0085] Synthesis of thioketone 4. Ketone 3 (4.4 g, 14.9 mmol) and
Lawesson's reagent (0.7 eq, 4.2 g, 10.4 mmol) were added to 500 mL
of toluene. The solution was heated to 80.degree. C. for 2 hours.
The dark green solution was allowed to cool to room temperature and
1200 mL of a 4:1 v/v mixture of hexanes and CH.sub.2Cl.sub.2 was
added. Filtration through a plug of silica gel and a small amount
of the same mixture of hexanes and CH.sub.2Cl.sub.2 was used to
wash the remaining product from the silica gel. 3 was isolated as a
green solid (2.2 g, 47%) after removal of the solvent and
triturating with cold hexanes.
[0086] Synthesis of diphenyldiazomethane 5a: A mixture of
4,4'-dihydroxybenzophenone (21.4 g, 100 mmol), 1-bromododecane
(49.8 g, 200 mmol), K.sub.2CO.sub.3 (50 g) in 500 mL of DMF were
heated with stirring at 120.degree. C. for 60 hours. After the
mixture was cooled to room temperature, 1 L of water was added. The
solution was extracted with CH.sub.2Cl.sub.2 (4.times.500 mL). The
combined organic layers were dried with MgSO.sub.4 and the most of
the solvent removed under reduced pressure as a white solid formed.
The solids were collected by filtration, washed with cold hexanes,
and air dried to give 4,4'-didodecyloxybenzophenone (41.2 g, 75%).
A mixture of 4,4' didodecyloxybenzophenone (20.6 g) and hydrazine
monohydrate (20 mL) in 150 mL of pentanol were heated at reflux for
24 hours. After cooling to room temperature, a white solid
precipitated which is collected by vacuum filtration, washed with
20 mL cold ethanol and air dried (19.4 g, 92%).
[0087] Synthesis of diaryldiazomethane 5b: To a mixture of compound
4,4'-didodecyloxybenzophenone hydrazone (10 g), yellow HgO (20 g)
in 150 mL of THF, 0.5 mL saturated sodium hydroxide in ethanol was
added. After stirring overnight the solution turned a dark purple
color and the solution was filtered. The resulting solution was
stored at -20.degree. C. for future usage.
[0088] Synthesis of olefin 6a: 1.1 eq. of diphenyldiazomethane 5a
dissolved in THF was added dropwise to a solution 1.0 g of
thioketone 4 in 100 mL of THF. The diphenyldiazomethane was added
until the green color of thioketone disappeared. After addition,
the reaction was stirred for 1 hr. The thioepoxide was obtained by
column chromatography (SiO.sub.2, 3:1 hexanes:CH.sub.2Cl.sub.2,
Rf=0.15) in quantitative yield (1.55 g, 100%). A solution of the
thioepoxide (1.55 g, 0.35 mmol) was then heated at reflux with
triphenylphosphine (1.01 g, 0.39 mmol) in 200 mL of anhydrous
p-xylene for 12 hours. After cooling to room temperature, the
solvent was removed under reduced pressure. The solid residue was
dissolved in 200 mL of CH.sub.2Cl.sub.2 and concentrated under
reduced pressure to 50 mL. Upon cooling on an ice/water bath, 6a
precipitates from solution. The solids were isolated by vacuum
filtration and washed with cold CH.sub.2Cl.sub.2. Compound 6a was
isolated as a white solid (1.35 g, 92%).
[0089] Synthesis of olefin 6b: To 1.0 g of thioketone 4 dissolved
in 100 mL of THF was added dropwise a solution of compound
diaryldiazomethane 5b until the green color disappeared. The
solution was stirred for 1 hr. The thioepoxide (1.79 g, 61% was
obtained by column chromatography (SiO.sub.2, 3:1
hexanes:CH.sub.2Cl.sub.2, Rf=0.15). A mixture of this thioepoxide
(895 mg, 1.06 mmol) and triphenylphosphine (334 mg, 1.27 mmol) in
100 mL of p-xylene were heated at reflux for 12 hours. The solvent
was removed under reduced pressure. Pure 6b (800 mg, 93%) was
obtained after column chromatography as a white solid (SiO.sub.2,
4:1 hexanes:CH.sub.2Cl.sub.2, Rf=0.20).
[0090] Synthesis of ketone 7a: KMnO.sub.4 (460 mg, 2.91 mmol) was
added as a solid to 6a (640 mg, 1.46 mmol) dissolved in 1 L of
acetone. The solution was stirred for 2 hours at room temperature
and then filtered. The solids were washed with 200 mL of
CH.sub.2Cl.sub.2. The combined organic solutions were washed with
800 mL of water. The phases were separated and the aqueous phase
was back extracted with CH.sub.2Cl.sub.2 (3.times.100 mL). Removal
of the solvent followed by column chromatography (SiO.sub.2, 45%
CH.sub.2Cl.sub.2 in hexanes, Rf=0.20) provided pure 7a (380 mg,
56.7%). The first fraction from the column was unreacted starting
material. (SiO.sub.2, 20% CH.sub.2Cl.sub.2 in hexanes,
Rf=0.20).
[0091] Synthesis of ketone 7b: Olefin 6b (250 mg, 0.31 mmol) was
dissolved in 500 mL of acetone and added KMnO.sub.4 (194 mg, 1.23
mmol). The mixture was stirred at room temperature for 2 hours. The
solution was filtered using vacuum filtration and the solids washed
with 200 mL of CH.sub.2Cl.sub.2. The organic washings were back
washed with 800 mL of water. The aqueous washings were extracted
with CH.sub.2Cl.sub.2 (3.times.100 mL). After removal of the
volatiles, the ketone (150 mg, 59%) was purified using column
chromatography (SiO.sub.2, 40% CH.sub.2Cl.sub.2 in hexanes,
Rf=0.20). The first fraction from the column was primarily starting
material (20% CH.sub.2Cl.sub.2 in hexanes, Rf=0.20).
[0092] Synthesis of thioketone 8a. Ketone 7a (757 mg, 1.65 mmol)
and Lawesson's reagent (368 mg, 0.91 mmol, 055 eq) were dissolved
in 300 mL toluene. The solution was heated for 30 minutes at
80.degree. C. After cooling to room temperature, the solution was
filtered through a plug of silica gel and washed with 4:1
hexanes:CH.sub.2Cl.sub.2. The solvent was removed under reduced
pressure to yield analytically pure compound 8a (774 mg, 1.63 mmol,
98%).
[0093] Synthesis of thioketone 8b: To 7b (147.4 mg, 0.178 mmol) in
80 mL toluene was added Lawesson's reagent (43.3 mg, 0.107 mmol).
The solution was heated for 30 minutes at 80.degree. C., and the
reaction was monitored by TLC. After cooling to room temperature,
the solution was filtered through a plug of silica gel and washed
with 4:1 hexanes:CH.sub.2Cl.sub.2. The solvent was removed under
reduced pressure to yield analytically pure compound 8b (150 mg,
98%).
[0094] Synthesis of bisolefin 2a: 1.1 eq. of diphenyldiazomethane
5a dissolved in THF was added dropwise to 8a (774 mg, 1.63 mmol) in
350 mL of THF. The diazomethane was added until the green color
disappeared. The reaction was stirred at room temperature for 1 hr.
The solvent was removed and the thioepoxide was purified by column
chromatography (SiO.sub.2, 3:1 CH.sub.2Cl.sub.2:hexanes, Rf=0.20)
to yield 928 mg, 1.45 mmol, 88%. A solution of this thioepoxide
(928 mg, 1.45 mmol) and triphenylphosphine (456 mg, 1.74 mmol) in
130 mL of anhydrous p-xylene was heated at reflux for 12 hours. The
solvent was removed under reduced pressure. Recrystallization from
2:1 v/v methanol:CH.sub.2Cl.sub.2 provided pure 2a (859 mg,
98%).
[0095] Synthesis of bisolefin 2b: To thioketone 8a (327 mg, 0.69
mmol) in 350 mL of THF was added dropwise a THF solution of the
diaryldiazomethane 5b until the green color disappeared. The
reaction was stirred for an additional hour. The THF was removed
under reduced pressure. The thioepoxide (600 mg, 0.60 mmol, 88%)
was obtained by column chromatography (SiO.sub.2, 3:1
CH.sub.2Cl.sub.2:hexanes, Rf=0.20). To this thioepoxide (600 mg,
0.60 mmol) in 100 mL of p-xylene was added triphenylphosphine (189
mg, 0.72 mmol). The mixture was heated at reflux for 10 hours. The
solvent was removed under reduced pressure. Pure compound 2b (510
mg, 0.52 mmol, 87%) was obtained by column chromatography (4:1
CH.sub.2Cl.sub.2:hexanes, Rf=0.20).
[0096] Synthesis of bisolefin 2c: A THF solution of the
diaryldiazomethane 5b was added to thioketone 8b (700 mg, 0.83
mmol) in 350 mL of THF until the green color disappeared. The
solvent was removed under reduced pressure. The thioepoxide as
obtained (928 mg, 0.67 mmol, 81%) by column chromatography
(SiO.sub.2, 3:1 CH.sub.2Cl.sub.2:hexanes, Rf=0.20). Pure compound
2c (859 mg, 98%) was obtained by column chromatography (4:1
CH.sub.2Cl.sub.2:hexanes, Rf=0.2). To this thioepoxide (928 mg,
1.45 mmol) in 130 mL of p-xylene was added triphenylphosphine (456
mg, 1.74 mmol). The mixture was heated at reflux for 12 hours. The
solvent was removed under reduced pressure. Pure compound 2c (859
mg, 98%) was obtained by column chromatography (4:1
CH.sub.2Cl.sub.2:hexanes, Rf=0.20).
[0097] Synthesis of contorted hexabenzocoronenes 1a: The photolysis
setup was performed in the well-known manner described in Liu, L;
Yang B; Katz, T. J; Poindexter, M. K. J. Org. Chem. 1991, 56,
3769-3775, the contents of which are incorporated by reference
herein. A mixture of compound 2a (500 mg, 0.82 mmol), iodine (965
mg, 3.78 mmol), propylene oxide (20 mL) in 350 mL of anhydrous
benzene were irradiated with UV light (Hanovia 450 W high-pressure
quartz Hg-vapor lamp) in an immersion well. Argon was bubbled
through the reaction vessel during the photolysis. To maintain a
constant temperature, the whole apparatus is submerged in a large
bath of circulating water. After 12 hours of irradiation, the
solvent is reduced to 15 mL under reduced pressure and a yellow
powder precipitates. Compound 1a is isolated by vacuum filtration
and washed with 100 mL of 20% CH.sub.2Cl.sub.2 in hexanes to yield
410 mg of 1a (83% yield).
[0098] Synthesis of contorted hexabenzocoronene 1b. A mixture of
compound 2b (510 mg, 0.52 mmol), iodine (602 mg, 2.35 mmol),
propylene oxide (10 mL) in 350 mL of anhydrous benzene were
irradiated with UV light in an immersion well. Argon was bubbled
through the reaction vessel during the photolysis. The whole
apparatus was submerged in a large bath of circulating water. After
12 hours of irradiation, the solvent is reduced to 15 mL under
reduced pressure and a yellow powder precipitates. Compound 1b is
isolated by column chromatography (SiO.sub.2, 4:1
hexanes:CH.sub.2Cl.sub.2, Rf=0.20).
[0099] Synthesis of contorted hexabenzocoronene 1c. A mixture of
compound 2c (394 mg, 0.29 mmol), iodine (340 mg, 1.33 mmol),
propylene oxide (20 mL) in 350 mL of anhydrous benzene were
irradiated with UV light in an immersion well. Argon was bubbled
through the reaction vessel during the photolysis. The whole
apparatus was submerged in a large bath of circulating water. After
12 hours of irradiation, the solvent is removed under reduced
pressure, and a yellow powder precipitates. Compound 1c is isolated
by column chromatography (SiO.sub.2, 4:1 hexanes:CH.sub.2Cl.sub.2,
Rf=0.25).
Example 2
Preparation of a Transistor Device Based on Contorted HBCs
[0100] Compounds of the formula 1c were spin-cast from
1,2-dichloroethane or CHCl.sub.3 to form uniform films
(approximately 100-nm thick) on top of a SiO.sub.2 substrate, and
then Au was deposited as source and drain electrodes by thermal
evaporation onto the spin-cast films through a metal-shadow mask. A
transistor device thus obtained is illustrated schematically in
FIG. 2. The transconductance and transistor output are shown in
FIGS. 10A and 10B.
[0101] The mobility (0.02 cm.sup.2 V.sup.-1 s.sup.-1) shown in FIG.
10B is calculated from the linear portion of the data in FIG. 10B,
and was based on a capacitance of 11.3 nF cm.sup.-2 for the gate
dielectric layer of 300 nm of SiO.sub.2 and a monolayer of
octadecyltrichlorosilane, obtained from a series of measurements
over a range of frequencies. Other critical parameters, such as the
threshold voltage for the device to turn on (as low as -3 V) and
the on/off current ratios in the device (106:1), are also very
good. These values are the best field-effect transistor properties
achieved for a columnar discotic material.
[0102] Several experimental techniques were used to determine the
presence of mesophases in the spin-cast film by compound. A
Differential Scanning Calorimetry (DSC) test showed an extra
transition (at around 90.degree. C.) other than the primary
transition temperature (at around 280.degree. C.), which indicated
the presence of an intermediate phase. (See FIG. 11A).
[0103] X-ray diffraction confirmed this intermediate phase. The
compounds of the formula 1c were heated above 295.degree. C. and
cooled, and data were collected upon cooling to 120.degree. C. in
FIG. 1B. The diffractogram is dominated by an intense reflection
characteristic of d.apprxeq.26 .ANG. (d is the distance between
neighboring lattice layers). The only other discernible features of
the diffractogram are two very weak, higher-order reflections (d=15
.ANG. and 13 .ANG.) that are indexed to a hexagonal arrangement of
columns. The lack of intensity in these higher-order reflections
indicates that the columns are not well correlated with each
other.
[0104] Further, polarized light microscopy revealed that these
columns in the mesophase aligned parallel to the surface. A sample
film was cooled to just below 278.degree. C. and captured as the
mesophase formed is shown in FIG. 11C. The birefringence of the
domains is extinguished when the column axes are aligned with the
polarizer or analyzer and is maximally bright when the stage is
rotated by 45.degree.. The micrograph is characteristic of a planar
arrangement of columns in a discotic mesophase shown schematically
in FIG. 11D.
[0105] NMR, UV-Vis, and fluorescence spectroscopic techniques were
also used to detect molecular association of the compound of
formula 1c in solution. The results indicate the aggregation of
this mesophase occurs in solution, and when a film is spin-cast
onto a transparent substrate, birefringent domains form which have
the same extinction as that described for the bulk film in FIG.
11C, suggesting that the film also has its columns aligned parallel
to the substrate.
Example 3
Fabrication of a Cut SWNT-Film Transistor Device
[0106] A compound of the formula 1c was spin cast onto the gap
between a cut SWNT such that it covered gap of the cut SWNT but did
not span the metal electrodes (shown in FIG. 15A). The device
obtained shows p-type hole transporting semiconductor behavior (see
FIG. 15B), but requires greater gate bias than a monolayer device
where R.sub.2 is COCl is used where surface attachment is
effected.
Example 4
Preparation of SWNTs and Metallic Electrodes
[0107] Individual SWNTs were grown by chemical vapor deposition
(CVD) using ethanol as the carbon source and CoMo-doped mesoporous
SiO.sub.2 catalyst particles patterned on thermally grown SiO.sub.2
layer on top of degenerately doped silicon wafers. The SWNTs
obtained were 1 to 2 nm in diameter.
[0108] Metallic electrodes (5 nm of Cr followed by 50 nm of Au)
separated by .about.20 .mu.m were then deposited through a metal
shadow mask onto the SWNTs using a thermal evaporator. The devices
thus fabricated can be conveniently tested using the metal pads as
source (S) and drain (D) contacts and the silicon substrate as a
back gate (G).
Example 5
Cutting the SWNTs
[0109] A slit window with a width of less than 10 nm was first
opened by ultrahigh-resolution electron-beam lithography on a
spin-cast layer of polymethylmethacrylate (PMMA) coated on top of
the SWNTs. Then an oxygen plasma (250 mTorr, 50 W RF power, 10 s
exposure) was applied to the open window to locally cut the SWNT
exposed. After development, the devices was washed by deionized
water and dried with a stream of N.sub.2 gas. The oxidation
reaction resulted in a prevalence of carboxylic acid groups on the
cut ends of the SWNTs.
[0110] Under a scanning electron micrograph, the gaps obtained are
too small to be observed. See FIG. 12A. However, Atomic Force
Microscopy (AFM) can readily locate and image the gap, as shown in
FIG. 12B, where the inset shows the height profile of the SWNT. The
size of the gaps is estimated to be not exceeding 10 nm in diameter
under the processing condition used above.
[0111] The electrical transport properties of the SWNT before and
after oxidative etching were measured to determine the yield of
completely cut tubes. Longer etch times give higher yields of the
cutting but lower the yields of the chemical connection reactions.
The etch time may be shortened so that the average gap can be
narrower than the window opened in the PMMA layer. Under the
processing conditions described above, .about.20 to 25% of the
tubes were completely cut among .about.2500 devices tested.
Example 6
Bridging the Ends of a SWNT Gap with Molecular Wires
[0112] Please refer to the relevant portion of Supporting Online
Material for: X. Guo et al., Science vol. 311, p. 356 (2006), for
instructions on the synthesis of diamine compounds of formulas
9-12, which is incorporated herein by reference in its
entirety.
[0113] The diamine compounds 9-12 shown in FIG. 7 were immersed
(individually or in certain combinations) together with the SWNT
devices obtained from Example 5 into a pyridine solution containing
carbodiimide dehydrating/activating agent EDCI, as explained in A.
Williams et al., Chem. Rev. vol. 81, p. 589 (1981), the contents of
which are incorporated by referenced herein. The diamine end groups
were chemically attached to the carboxylic acid by a dehydration
reaction to form the amide linkage. After reaction, the devices
were removed from the solution, rinsed with fresh solvent, dried,
and then tested electrically.
[0114] These bridging molecular wires also allow for calibration of
the etch process itself because the different species can be used
as molecular rulers. For example, under optimized conditions, the
yield for connection of compound 9 for more than 200 reactions is
10%. Using longer etch times, which give the larger gaps, reduces
the yield of the coupling reaction with compound 9. Moreover,
molecules of length similar to that of compound 9 give similar
yields, implying that the yield is dominated by the statistics of
having two functional groups appropriately spaced for bridging.
Under identical conditions, the longer molecules (10 and 11) gave
lower yields in their connection reactions (5%). A mixture of three
oligomers based on compound 10 that ranged in length from 2 to 6 nm
make the yield increase to 20%.
[0115] Electrical measurements on devices before cutting, after
cutting, and after connection with compound 9 are shown in FIG. 13.
FIG. 13A shows drain current (I) in the device made by metallic
SWNT electrodes connected with compound 9 as a function of the gate
voltage (V.sub.G) at source-drain voltage V.sub.SD=50 mV. FIG. 13B
shows drain current as a function of V.sub.SD with no gate bias.
FIG. 13C shows drain current as a function of V.sub.G at
V.sub.SD=50 mV for a semiconducting SWNT connected with compound 9.
Before cutting, the device in FIGS. 13A and 13B shows metallic
behavior, and the device in FIG. 13C shows typical p-type
semiconducting behavior. After cutting, neither of the two devices
show conductance (i.e., the conductance is at background noise
level of the measurement (.about.2.0 pA). After molecular
connection of the SWNT leads, the two devices recovered their
original metallic or semiconducting behavior at reduced values of
I, indicating that the gate modulates the nanotube conductance more
strongly than that of the molecules. Similar I-V.sub.SD curves are
obtained for measurements of other molecular bridges (compound 10
in FIG. 13D and compound 11 in FIG. 13E). The resistance of the
molecular wires, or the molecular conductance, may be estimated
from the drop in current after molecular connection.
Example 7
Fabrication of a Cut SWNT-Monolayer Device
[0116] The open SWNT circuits were immersed in a THF solution of a
compound of the formula 1, wherein R.sub.1 is H and R.sub.2 is COCl
and a compound of the formula 1, wherein R.sub.1 is
OC.sub.12H.sub.25 and R.sub.2 is --COCl. The devices were removed
from solution, rinsed, and dried under a stream of inert gas.
UV-Vis spectroscopy, surface X-ray scattering, and florescence
spectroscopy characterized the layer grown at the SWNT gaps as
densely packed monolayers on silicon oxide on the surface of
silicon wafers.
[0117] Monolayers of both compounds behave as p-type semiconducting
films. FIG. 14 shows the transistor characteristics for a monolayer
of a compound of the formula 1, wherein R.sub.1 is
OC.sub.12H.sub.25 and R.sub.2 is --COCl assembled in the same
device characterized in FIG. 8. FIG. 14A shows the transistor
output, V.sub.G=0 to -5V in 1V steps (corresponding from bottom to
top to the curves in FIG. 14A); FIG. 14B shows transfer
characteristics for the same device at V.sub.D=-2V. The ON/OFF
current ratios of these devices are high (.about.5 orders of
magnitude). This ratio is one of the critical parameters for the
success of nanoscale organic field effect transistors and has
proven difficult to optimize in ultrasmall organic Field Effect
Transistors (FETs) with size smaller than 10 nm prepared with
either metal or SWNT electrodes. The devices made with these
contorted HBCs and SWNTs require much lower gate bias to switch
(over an order of magnitude smaller compared to the gate bias
required to switch other organic thin film materials).
Example 8
Detection of an Electron Rich Molecule
[0118] A device illustrated in FIG. 8, wherein the self-assembled
monolayer of a compound of the formula 1, wherein R.sub.1 is
OC.sub.12H.sub.25 and R.sub.2 is --COCl is used as the molecular
wire, was dipped into a solution of an electron acceptor
.alpha.,.alpha.,.alpha.',.alpha.'-tetracyano-p-quinodimethane
(TCNQ). Electric characteristics of the device were measured both
before and after the dipping. FIGS. 16A and 16B show the transistor
characteristics for the same device shown in FIGS. 14A and 14B
where the monolayer is made by the compound. 10A plots the
transistor output at V.sub.G=0 to -5V in 1V steps; FIG. 16B plots
the transfer characteristics for the device, V.sub.SD=-2V. After
contact with TCNQ, the OFF-current increases by roughly an order of
magnitude and the ON-current increases slightly, and the threshold
voltage shifts. An uncut metallic SWNT device shows no effect when
dipped into a solution of TCNQ, indicating that the change observed
in FIGS. 10A and 10B is due to the interaction of TCNQ with the
self-assembled monolayer.
Example 9
Detection of pH Change in a Medium
[0119] A device fabricated according to FIG. 4 using compound 12 as
the molecular bridge was subjected to a series of switching between
a solution with pH=3 and a solution with pH=11. The current at
saturation was monitored at the end of each cycle after the device
was rinsed, dried, and tested. The response to changes in pH for
the device is shown in FIG. 17. The molecular conductance varied
significantly at different pH, i.e., nearly an order of magnitude,
from .about.5.2.times.10.sup.-4 e.sup.2/h at pH=3 to
.about.5.0.times.10.sup.-5 e.sup.2/h at pH=11 over many switching
cycles.
Example 10
Detection of a Biological Binding Event
[0120] A device based on a cut SWNT and a molecular bridge of a
compound of the formula 13, a fluorenone molecule was fabricated,
whose structure is illustrated schematically in FIG. 18A. The
fluorenone bridge was functionalized by Biotin (biotinylated
alkoxylamine) through an oxime formation, as illustrated in FIG.
18B. The device was then exposed to Streptavidin which is attached
to a gold bead of diameter .about.5 nm, and the electric
characteristics before and after the exposure was measured. The
strong affinity between Biotin and Streptavidin produced noticeable
alterations of the electric characteristics of the device, as
reflected in large shift in the threshold voltage and an increase
in the low-field conductance shown in FIG. 18C. An AFM micrograph
of the device in the SWNT gap area (FIG. 18D) shows the presence of
the gold bead attached to the Streptavidin due to the
Streptavidin-Biotin binding at the gap.
[0121] As another variation of detecting a biological binding
event, a fluorenone bridge molecule 14 was utilized to attach an
antibody to the bridge. In this case, the anti-FLAG antibody that
was synthetically modified in the labs of Professor Matthew Francis
from the Dept. of Chemistry at UC Berkeley was utilized. Before
attachment the antibody was modified with a ketone. To attach this
to the molecular bridge, a bisalkxoylamine tether that was first
reacted with the single molecule device 14 wired in between
semiconducting SWNT leads to yield device 14A. As shown in FIG. 19,
14A presents an alkoxylamine function for the sequestering of the
modified antibody to yield 14B. We next tested the changes in
conductance when the device is exposed to the FLAG peptide sequence
to produce 14C. The antigen can then be removed from the device to
return to 14B by incubation with the unattached anti-FLAG antibody.
This cycle is shown schematically in FIG. 19. The preliminary data
of FIG. 19C shows a factor-of-five change in the on-state
conductance with antigen binding.
[0122] The foregoing merely illustrates the principles of the
invention. Various modifications and alterations to the described
embodiments will be apparent to those skilled in the art in view of
the teachings herein. It will thus be appreciated that those
skilled in the art will be able to devise numerous techniques
which, although not explicitly described herein, embody the
principles of the invention and are thus within the spirit and
scope of the invention.
* * * * *