Supplemental Information for APL Manuscript No. MS #L12-12176
Field-effect-based chemical sensing using
nanowire-nanoparticle hybrids: The ion-sensitive
metal-semiconductor field-effect transistor
Vivek Pachauri*, Klaus Kern and Kannan Balasubramanian*
[Brief description]: Fabrication and characterization of ZnO ISMESFETs and the analysis of
field-effect behavior of nanowires-nanoparticle hybrids using Schockley model.
Fabrication of ZnO NW devices
The ZnO NW FETs were fabricated using a site-specific self-assembled solution based
growth method reported by us previously. [1] Aqueous solutions of zinc nitrate
[Zn(NO3)2.6H2O] and hexamethylenetetramine [(CH2)6.N4] are used as precursors for the
growth of ZnO nanowires in 1:1 molar ratio (0.25mM). The growth was carried out for 25
hours.
The
electrodes
and
the
microchannels
were
fabricated
by conventional
photolithography techniques at the Institute for Microtechnology (HSG-IMIT, VillingenSchwenningen, Germany). The substrates were sized 4 × 4 mm equipped with four open
microfluidic channels etched using reactive ion etching. Each of the microfluidic channels is
30 microns wide and terminates in a reservoir on each end. Each microfluidic channel is
provided with two source-drain microelectrode pairs exposed only in the middle of the
channel. This architecture is used in order to bring the analyte solution from the reservoirs
towards nanowire devices by capillary action. After fabrication, the ZnO NW devices are
subjected to rapid thermal annealing (RTA) in air at 8000C for 25 seconds. Rapid thermal
annealing improves the nanowire-metal contacts and removes the oxygen vacancies present in
the ZnO nanowire crystals as suggested by earlier studies.[2,3]
Fabrication of ZnO NW ISMESFETs
m- NPs gates on the ZnO NW devices were deposited in a controlled electrochemical method.
Figure 2C showed schematics of an electrochemical cell which consisted of a microwell with
Pt counter and pseudo reference electrodes and the contacted ZnO NWs acting as the working
electrode. Gold and palladium NPs were chosen due to their high work functions and the
possibility to form Schottky contacts with ZnO.[4] The gold and palladium NPs were
deposited from an aqueous solution prepared by mixing lithium perchlorate (0.1M LiClO4)
with 2mM HAuCl4 and 2mM Na2PdCl4 respectively.[5] Lithium perchlorate works as a
background electrolyte. The NPs are formed with potentiostatic control at a voltage of -0.7V
to -0.5V versus a Pt pseudo reference electrode. The deposition time varied from 8 seconds to
15 seconds for optimal NP coverage of the ZnO nanowires. The devices were taken out
immediately of the electrolyte solution after the deposition, washed in deionized water and
blow dried in nitrogen before carrying out any measurements.
Device characterization
The nanowires devices were characterized by scanning electron microscopy (SEM). SEM
images were obtained on a Gemini Ultra 55 operating at 4 to 5 kV. Atomic force microscopy
(AFM) images were obtained using a Digital Instruments Dimension 3000 operating in
tapping mode. The size of the NPs was found to be in the range of 60 to 80 nm as inferred
from AFM profiles (figure S1). The fabricated ZnO nanowire devices were characterized
using a home-made transport measurement system comprising a 2 channel voltage source
(Keithley 2602 for drain bias and gate voltage), a current-to-voltage converter and a Keithley
2000 multimeter. A manual wafer probe system (Süss Microtec PM5) with probe heads and
probe needles was used to contact the pads leading to the electrodes. The field-effect behavior
of the devices was analyzed in a liquid gating set up.[6] The field-effect measurements before
and after the deposition of nanoparticle were carried out in buffer solutions. All buffer
solutions (pH 7, 8 and 9) were prepared by using an appropriate mixture of Na2HPO4 and
citric acid (0.2 M and 0.1 M respectively) to attain different pH values. The chemical
functionalization of the NW-NP hybrids was carried out by immersing NP decorated devices
in 1 percent ethanol solutions of p-ATP and p-HTP for 45 minutes typically. Short times for
functionalization are used to ensure minimum non-specific binding of the thiolated receptors
on the ZnO and SiO2 surfaces. The devices were thoroughly washed in ethanol after the
chemical functionalization.
Figure. SI1: Structural characteristics of a typical ZnO NW MESFET as observed under an atomic force
microscope (AFM). Height profiles on the top right and bottom right give an idea of the diameter of ZnO NW
and size of palladium NPs (~ 70 nm in this case)
Analysis of NP-NW junction using Schockley model:
Metals with high work functions such as Gold, Silver, Platinum, Palladium etc. form highly
rectifying junctions when brought in contact with n-type semiconducting material. The larger
work function of m-NPs than the semiconductor electron affinity , causes a flow of electrons
from the semiconductor NW to the NPs in order to align the Fermi levels. The remaining
positively charged atoms in the NW create a space charge region which forms a barrier
of height ΦBn for electron flow from NWs to NPs.
ΦBn = (Φm - χ)………………………………………………………………………..(1)
(Where Φm is the work function of the m-NPs and χ is the electron affinity)
The built in potential (eVbi) induces a depletion layer of width ‘w’ in the semiconductor NWs.
eVbi = ΦBn – (EC - EF)………………………………………………………………. (2)
Applying a positive bias shrinks the depletion layer width and forward bias the junction while
a negative bias (VG) thickens the depletion layer and at sufficiently negative bias shuts off the
channel completely. This voltage is called the threshold voltage (Vth) of the device:
Vth = Vbi – qNd2 / 2εs ………………………………………………………………. (3)
Here, εs is the permittivity of the semiconductor, q is electron charge, Vbi is the voltage at
Schottky junction and N is the density of electrons in the material. [7] The threshold voltage
depends on the built-in potential and the channel thickness d. The threshold voltage can also
be seen as function of the pinch-off voltage Vp:
Vth = Vbi – Vp ……………………………………………………………………
(4)
In order to explain the enhanced field-effect obtained for ZnO NW MESFETs, let us begin
with a simple model of MESFET considering a planar device with n-type channel covered
with metal gate as shown in figure SI2. In the case of device (a), the channel is pinched off by
a planar portion of depletion layer. In the Shockley model analysis for pinch-off in a planar
depletion layer is given byVPO = qNDa2/2εr ε0 ……………………………………………………..………… (5)
(Where q is electron charge, ND is electron density, a is channel depth, εr permittivity of the
medium and ε0 is permittivity of free space.)
So the pinch-off current is directly proportional to the area of depletion layer. In the case of
small gate lengths, the formation of depletion layer is spherical and pinch-off voltages are
calculated combining Gauss’s law:
VPO(spherical) = - q ND/ 2εr ε0[rd2ln·rd/rj – (rd2- rj2)/2]............................
(6)
Equation 5 and 6 give the peak-off voltages for the devices with lG ~ lCh and lG << lCh.
Ratio of the two pinch-off voltages in device (a) and device (b) can be established now:
VPO (spherical) / VPO (parallel) = rd2/ a2.ln rd/ rj – (rd/a)2. ½[1-( rj/rd)]
Since at pinch-off, rd = a and junction depth is very small than rd so rd/ rj = > 1
Hence VPO(spherical) > VPO(parallel)
Thus, pinch-off voltages for device model with spherical charge depletion layer are calculated
to be higher than that of device model with planar charge depletion layer. Higher pinch-off
voltages lead to higher saturation currents which is related to transconductance by the
equationID = G0 (1-h/a) Vds (where ID is saturation current and G0 is transconductance)
or, G0 = ID/ Vds(1-h/a) α VPO
Figure. SI2: Schematic comparing the formation of depletion region in (a) a planar metal
semiconductor contact where channel length lch ≈ gate length lg and (b) the formation of a
spherical depletion region in a metal semiconductor contact where channel length lch < gate
length lg. (a) represents the situation of classical MESFETs, while (b) represents the situation
of our NP decorated ZnO NW devices.
Figure. SI3: Field-effect behavior of a typical ZnO NW device before and after
electrochemical modification showing negligible hysteresis (~ 15 mV) between forward and
backward sweeps. This justifies the interpretation of the large observed threshold shifts (>
60mV) as arising due to a real sensor response.
References for Supplemental Information:
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