Electrostatic Limit of Detection of Nanowire-Based Sensors

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Electrostatic Limit of Detection of Nanowire-Based Sensors Alex Henning , * Michel Molotskii , Nandhini Swaminathan , Yonathan Vaknin , Andrey Godkin , Gil Shalev , and Yossi Rosenwaks

1. Introduction

Chemical and biological solid-state sensors with a 1D confi ne-

ment, in particular nanowire (NW) based fi eld-effect transis-

tors (FETs) [ 1–11 ] of various materials and carbon nanotubes

(CNTs) [ 12–14 ] were demonstrated as ultra-sensitive sensors. In

general, the principle of operation involves an analyte that

is adsorbed on the surface of the 1D structure and induces a

change in the electrostatic surface potential (i.e., fi eld effect),

which in turn entails a current or capacitance modulation.

Detection limits down to parts per billion (ppb) [ 4,14 ] and even

parts per trillion (ppt) [ 5 ] were demonstrated with CNT and

NW FETs for NO 2 and TNT, respectively. Femtomolar detec-

tion limits were demonstrated in liquid environment with

NW biosensors. [ 3,15,16 ] The limit of detection (LOD), com-

monly accepted as the analyte concentration that produces

a signal higher than three times the noise level, is a critical

DOI: 10.1002/smll.201500566

Scanning gate microscopy is used to determine the electrostatic limit of detection (LOD) of a nanowire (NW) based chemical sensor with a precision of sub-elementary charge. The presented method is validated with an electrostatically formed NW whose active area and shape are tunable by biasing a multiple gate fi eld-effect transistor (FET). By using the tip of an atomic force microscope (AFM) as a local top gate, the fi eld effect of adsorbed molecules is emulated. The tip induced charge is quantifi ed with an analytical electrostatic model and it is shown that the NW sensor is sensitive to about an elementary charge and that the measurements with the AFM tip are in agreement with sensing of ethanol vapor. This method is applicable to any FET-based chemical and biological sensor, provides a means to predict the absolute sensor performance limit, and suggests a standardized way to compare LODs and sensitivities of various sensors.

Limit of Detection

A. Henning, Dr. M. Molotskii, N. Swaminathan, Y. Vaknin, A. Godkin, Dr. G. Shalev, Prof. Y. Rosenwaks Department of Physical Electronics School of Electrical Engineering Tel-Aviv University Ramat-Aviv 69978 , Israel E-mail: [email protected]

sensor parameter that is often confused with the sensitivity,

defi ned as the response signal per unit concentration. [ 16–18 ]

However, the LOD of a given sensor is exceedingly dif-

fi cult to determine because it is analyte dependent, requires

reliable low analyte concentrations, and is diffi cult to com-

pare in actual sensing experiments due to variations of the

reaction dynamics, [ 19 ] surrounding environment, carrier gas

or solution, and calibration standard of the reference sensor.

We present a method based on scanning gate microscopy

(SGM) to determine the electrostatic LOD (eLOD) that is

independent of the analyte and its reactivity with the sensor

surface, and therefore provides a standardized way to com-

pare chemical sensor performance. As FET-based sen-

sors are susceptible to surface induced charge, the eLOD

determines the detection limit in terms of charge, i.e., the

total induced charge that produces a sensor response signal

higher than three times the signal noise level. SGM is an

atomic force microscopy (AFM) based method where the

AFM tip is used as a local gate to control the conductance

of the underlying sample. SGM was used to study thin fi lm

transistors, [ 20 ] graphene, [ 21 ] and the electronic transport in

individual CNTs, [ 22,23 ] localize trap centers of nanotube

quantum dots, [ 24 ] characterize InAs NWs, [ 25 ] and Schottky

barrier FETs. [ 26 ] We use SGM to determine the eLOD of a

NW-based chemical sensor with a precision of subelemen-

tary charge. By using the AFM tip as a local front gate, we

small 2015, DOI: 10.1002/smll.201500566

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emulate the fi eld effect of surface potential changes induced

by the binding of target molecules onto the gate-oxide sur-

face of an FET-based sensor. We quantify the tip induced

surface and interface charges using an analytical model and

show that this model is in good agreement with fi nite element

calculations and matches the outcome of electrostatic device

simulations. Furthermore, the presented method is validated

against actual gas sensing of ethanol vapor. We demonstrate

that a typical silicon NW sensor is sensitive to the presence

of a single elementary charge. This method is generally appli-

cable to any FET-based chemical and biological sensor and

allows for a comparison of these sensors independently of

the analyte and its interaction with the sensor surface.

2. Results and Discussion

The SGM technique to experimentally determine the eLOD

of NW-based sensors is demonstrated with the electrostati-

cally formed nanowire (EFN) sensor (see Figure 1 ), which is

a variant of conventional NW-based sensors. This particular

EFN platform was selected due to its large dynamic range

and high sensitivity as the diameter is electrostatically adjust-

able. [ 27 ] Furthermore, the planar and well-defi ned structure of

this device facilitates 3D modeling. The EFN is a nanowire-

like conducting channel that is electrostatically formed by

appropriate biasing of a multiple gate FET. It is fabricated

from a silicon-on-insulator wafer that supports the presence

of a back gate, V BG . The EFN is composed of an n-type doped

(4 × 10 17 cm −3 ) silicon region that is ≈500 nm in width and

is fl anked by two lateral p-type regions (2 × 10 20 cm −3 ). The

electron-accumulated conducting channel fl ows inside the

n-type region and the width of the channel can be reduced

by reverse biasing the p-n junctions using the lateral junc-

tion gates, V JG1 and V JG2 . The analytes adsorb on the active

sensing area located on top of the n-type silicon, alter the

surface charge density and consequently the surface poten-

tial, and force depletion or accumulation in the underlying

NW and by this means modulate the drain current, I D . The

electron-accumulated channel between source and drain is

confi ned to a nanowire (dark blue channel in Figure 1 ) with

particular shape and effective diameter, d eff , of the cross-sec-

tional area by appropriate biasing of the surrounding gates,

V JG1 , V JG2 , and V BG . The active sensing area is passivated with

a 6 nm thick thermal SiO 2 that can be modifi ed, for instance,

with a self-assembled monolayer, in order to achieve a higher

selectivity.

A schematic illustration of an SGM confi guration is also

shown in Figure 1 . The platinum covered AFM tip with an

apex radius of R ≈ 30 nm is placed with nanometer precision

above the NW. Depending on the polarity of the tip voltage,

V tip , the tip induces either a depletion ( V tip < 0 V) or accumu-

lation ( V tip > 0 V) of electrons in the EFN channel. To avoid

permanent and irreversible charging of the SiO 2 top layer, we

performed the SGM measurements in intermittent contact

mode (“tapping”) and not in contact mode.

Figure 2 a presents a characteristic contact potential dif-

ference (CPD) map of the EFN conducting channel that was

measured with Kelvin probe force microscopy (KPFM). The

EFN cross-sectional diameter was extracted from the meas-

ured CPD of an EFN device under operation using Ohm’s

law, described in detail in previous reports. [ 27,28 ] KPFM is

an AFM-based technique capable of measuring the work

function difference, Δ φ, between probe tip and sample with

small 2015, DOI: 10.1002/smll.201500566

Figure 1. An illustration of the scanning gate microscopy confi guration with a multiple gate sensing device. By applying voltages to the two junction gates ( V JG1 and V JG2 ) and the back gate ( V BG ), a nanowire-like channel (dark blue) is formed inside the n-doped silicon region along the y -axis of the device. The silicon layer, with a thickness of 150 nm, is on top of the back oxide. The active sensing area is covered with a 6 nm thick thermally grown SiO 2 layer.



nanometer spatial resolution and meV sensitivity. [ 29, 30 ] It was

found that the effective NW diameter is tunable in a range

from 115 to 22 nm by reducing the two junction gate biases,

V JG12 , from 0 to −2 V. Note that the CPD image in Figure 2 a

resolves the p-doped regions (red) and the n-doped channel

(blue) through the SiO 2 top layer. Figure 2 b shows SGM

measurements where the drain current was measured as a

function of the tip position (indicated by dashed arrows in

Figure 2 a) at V tip = −1 V for an effective channel diameter of

115 nm ( V JG12 = 0 V). The tip has the strongest effect (local

depletion) on I D when it is placed in the lateral and trans-

versal center of the NW. Local variations on a sensor surface,

e.g., in NW-based bio(chemical) sensors where an optimal

placement of receptors is important to achieve a high sensi-

tivity, [ 26 ] can be identifi ed due to the high precision of the tip

gate positioning.

Figure 3 a shows CPD images of the EFN sensing

area under device operation before and after exposure

to ≈2000 ppm of ethanol, and demonstrates the “molec-

ular” gating effect of the adsorbed ethanol on the EFN.

The adsorbed ethanol molecules induce a CPD (= −surface

potential) shift of ≈50 mV in the p-type region and lead to

an overall CPD increase of ≈180 mV in the n-doped EFN

channel region, as shown in the CPD profi les along the

p-n-p region in Figure 3 b. This positive CPD change is a

direct evidence that the surface is negatively charged upon

ethanol adsorption due to surface polarization, in agreement

with previous work. [ 31 ] This negatively charged layer acts as

a molecular gate causing depletion in the n-doped region

and resulting in a more positive CPD (more p-type) of the

nanowire. The highly complex interaction of ethanol with an

SiO 2 surface was studied in previous works, e.g., by Chang

and Shu. [ 32 ] KPFM measurements are supported by I D – V D

characteristics of the EFN device (Figure 3 c) showing the

decrease in I D following ethanol adsorption.

The complete EFN device was simulated with a 3D

device simulator (Synopsys TCAD Sentaurus, Mountain

View, USA) taking into account the actual process

parameters, where for each mesh point the Poisson and con-

tinuity equations were numerically solved. Figure 4 a shows

a semilogarithmic plot of the measured I D (red squares)

as a function of the applied tip voltage, V tip , and the simu-

lated I D (black triangles) as a function of the fi xed charge

inside a 10 3 nm 3 SiO 2 cube placed on top and in the center

small 2015, DOI: 10.1002/smll.201500566

Figure 2. a) Contact potential difference (CPD) image of an EFN under operation for V D = 1 V and V JG12 = 0 V. The dashed arrows indicate the scanning lines across the p-n-p junction in scanning gate microscopy. b) The 3D plot shows the drain current as a function of the tip position on the X - and Y -axis for V tip = −1 V. The tip–sample distance, δ , was kept constant at ≈15 nm.

Figure 3. a) CPD images of the EFN active area before and after ethanol exposure (≈2000 ppm) under device operation at V D = 1 V and V JG12 = −0.5 V. b) CPD line profi les, indicated by dashed arrows in (a), across the p-n-p junction of the EFN before and after ethanol adsorption. Ethanol alters the surface potential causing a depletion of the conductive channel in the n-doped region refl ected in the more positive CPD compared with the bare EFN. c) The corresponding I D – V D characteristics show a decrease in I D by a factor of ≈2 following ethanol exposure.



of the active sensing area. The measurements and simula-

tions were performed for two NW diameters with distinct

sensitivity and large dynamic range, d eff of 80 and 30 nm by

applying V JG12 = −0.5 V and V JG12 = −1.5 V, respectively. The

measured I D – V tip curves are reminiscent of the transfer

characteristics, I D – V G , of a conventional FET, which can

be switched on and off by appropriate biasing of the gate

voltage, V G . The simulations are in accordance with the

measured I D – V tip characteristics: A negative (positive) tip

voltage is equivalent to a negative (positive) charge placed

on top of the sensor, which closes (opens) the channel

(EFN). It is evident from the plot in Figure 4 a that the effect

of the tip gate on the drain current, expressed in terms of

sensitivity, S = ΔI D / I D , is greater for smaller d eff . For instance,

we obtain a higher sensitivity, S 30 nm = 0.27 > S 80 nm = 0.1, for

the NW with d eff = 30 nm than for the NW with d eff = 80 nm

( V tip = −1 V) although the active area is larger for the latter

providing more binding sites. This result is refl ected in the

higher on/off ratio (10 2 –10 3 ) of the 80 nm wide EFN com-

pared with the lower on/off ratio (<10) of the 30 nm narrow

EFN. This fundamental result is in agreement with ethanol

gas sensing experiments (Figure 4 b), where the EFN sensor

was exposed to ethanol concentrations in the range of

10–2000 ppm for NWs with diameters of 80 and 30 nm at

a negative V BG . The increase in sensitivity with decreasing

d eff is explained by the infl uence of the local electric fi eld

that stems from a change in the surface charge density

due to adsorbed analyte (ethanol here) or the tip on the

electron density distribution inside the NW; smaller NW

is more affected by a local electrostatic fi eld. The Debye

screening length was calculated for the n-doped silicon with

the expression, DB

2d

L k Tq Nε= , for semiconductors where k B

is the Boltzmann constant, T is the temperature, ε is the die-

lectric constant, and N d is the donor density of the n-type

region, resulting in L D ≈ 6. Therefore, induced surface

charges are partially screened. The tip gate measurements

can predict the lower limit of a minimum detectable surface

charge and are in qualitative agreement with ethanol gas

sensing experiments. However, for a quantitative comparison

the tip induced surface charge has to be translated into

an analyte concentration. This requires information on the

interaction of the analyte with the surface, in particular the

binding equilibrium kinetics and surface coverage.

We have calculated the tip induced surface and interface

charges for the probe geometry shown in Figure 5 adapting

the general analytical solution for the electrostatic fi eld of

a point charge, Q , in a three-dielectric medium derived by

Barrera et al. [ 33 ] using the method of images. Instead of a

point charge, we considered the electrostatic fi eld and sur-

face charge density of an AFM tip. The detailed calculation

is provided in the Supporting Information. For tip–sample

distances in the range of (or larger than) the apex radius, the

contribution of the cone becomes signifi cant. [ 34–36 ] As illus-

trated in Figure 5 , the tip is modeled as a truncated cone with

a semispherical tip apex of radius R at a tip–sample distance

small 2015, DOI: 10.1002/smll.201500566

Figure 4. a) The tip voltage is shown as function of the measured drain current, I D , for two different channel diameters, 80 and 30 nm, in a semilogarithmic plot, and compared with the simulated I D plotted as a function of the fi xed charge placed above the NW center. The applied tip voltage, V tip , was swept from −10 to 10 V at a fi xed tip position ( δ ≈ 15 nm) above the active sensing area. V D = 1 V and V BG = 0 V. b) I D is plotted as a function of the ethanol vapor concentration that was varied from 10 to 2000 ppm, at V D = 1 V and V BG = −3 V in a sensing experiment for two different channel diameters, 80 and 30 nm.

Figure 5. A schematic representation of the tip above the sample surface showing the dimensions and parameters used in the analytical model that was solved to quantify the tip induced surface and interface charges, q 32 and q 21 , respectively. L , r , and θ are the cone length, radius, and half-cone angle, respectively. σ cone and σ apex are the surface charge densities of tip cone and apex, respectively. The tip apex with radius R is located in a medium with dielectric constant ε 3 (nitrogen) at a tip–sample distance δ . The surface top layer with thickness t and dielectric constant ε 2 (SiO 2 ) covers the bulk material with dielectric constant ε 1 (silicon).



δ . [ 34 ] Terris et al. [ 37 ] investigated the charging of an oxidized

Si surface using AFM and suggested a spherical model for

the tip, where the capacitance is modeled by a sphere above

a conductive plane, 4apex 01

1

C Rsinh sinhnn∑πε α α( )= −

=

∞, where

11cosh Rα δ( )= +− . The electric fi eld of the cone is approxi-

mated by the fi eld generated by a charge line of constant

charge density, 411cone 0 tip

1

V ln coscosλ πε θ

θ{ }( )≈ +−

−, suggested by

Hao et al. [ 38 ] The tip induced surface charge densities were

obtained expanding the solution of Barrera et al. [ 33 ] using the

expression for the tip apex charge, Q apex = V tip C apex , and infi n-

itesimal cone charge, dd

11

conetip

1Qs V ln cos

cosθθ{ }( )= +

−

−, where s

is the distance to the center of curvature of the apex. The

total fi eld of the tip and consequently the total induced sur-

face and interface charge density is the sum of conical and

spherical contributions, tot ap coij ij ijσ ρ σ ρ σ ρ( ) ( ) ( )= + , where

ρ is the radial coordinate in a cylindrical coordinate system

with azimuthal symmetry and ij = 32 and ij = 21 indicate the

surface and interface plane, respectively. The total charge

(at V tip = 1 V) induced on the entire (infi nite) surface and

interface planes, q 32 and q 21 , respectively, is obtained by

solving the integral equation of the respective charge densi-

ties, σ ij , for an infi nite plane

2 d

0

q ij ij∫ρ π ρσ ρ ρ( ) ( )=∞

(1)

Each tip voltage corresponds to a specifi c charge quantity,

which is simply obtained by multiplying q ij ( ρ ) with V tip due its

linear relationship. Figure 6 shows the calculated charge den-

sity, tot32tot

21totσ ρ σ ρ σ ρ( ) ( ) ( )= + , in a contour plot (Figure 6 a)

and in a 3D representation with logarithmic length scales

(Figure 6 b) to elucidate the entire infl uence radius of the

tip induced charge densities at V tip = 1 V. By solving Equa-

tion ( 1) the total bound surface charge induced on the

entire surface and interface plane is q 32 ( ρ ∞ ) = 3.37 × 10 −16 C

and q 21 ( ρ ∞ ) = 7.74 × 10 −17 C, respectively. These results

were verifi ed by fi nite element calculations (Figure S2,

Supporting Information) of the probe charge resulting in

q tip = 6.88 × 10 −16 C, which is expected to be higher (by a

factor of ≈1.5) than the tip induced charge because the die-

lectric SiO 2 was neglected. Figure 6 also shows that the tip

induced charge density rapidly drops with the radial distance,

ρ , in quantitative agreement with the SGM results presented

in Figure 2 b where the infl uence of the tip on the drain cur-

rent drops to zero within a length scale of 1 µm. Since the

active area of our sensor is electrostatically confi ned to a

NW with an effective width, d eff < 100 nm, we calculated the

induced charge for a fi nite areal distribution (light blue stripe

in Figure 6 a).

We use the above analytical model in order to esti-

mate the eLOD of the electrostatic NW. The minimum tip

voltage (≈50 mV) that still has an effect on the drain current

of the device at optimum working conditions ( d eff = 30 nm

and V JG12 = −1.5 V) corresponds to a tip induced charge

(1.1 ± 0.3 e ) that we hereby defi ne as the eLOD of the chem-

ical sensor (see Figure 7 ). A comparison between the meas-

ured eLODs of two NWs ( d eff of 80 and 30 nm) is provided

in Table 1 . The error arises mainly due to variations in the

tip–sample distance (<1 nm) caused by uncompensated elec-

trostatic forces.

By using the relation, ΔI D = g m × Δ Ψ, [ 16 ] between the

transconductance with respect to the molecular (top) gate,

g m , the surface potential change, Δ Ψ, and the drain current

change, ΔI D , we can estimate Δ Ψ. For the 30 nm EFN device

the measured values are g m = 4.6 × 10 −7 S (see Figure S5, Sup-

porting Information) and I D = 2.5 × 10 −10 A (see Figure 7 ),

and the LOD can be expressed as a minimum surface poten-

tial change, LOD Ψ ≈ 0.5 mV, which is in good agreement with

the result (LOD Ψ ≈ 1–2 mV) reported by Chen et al. [ 39 ]

The sensitivity of our method is limited by the minimum

detectable dc voltage change, ΔV tip = 10 mV, of the measure-

ment instrument. Each tip voltage corresponds to a charge

equivalent; for the case of the NW with d eff of 30 nm and

area of 0.15 µm 2 we can adjust the tip induced charge with

a precision of 0.2 ± 0.1 e . Due to a work function difference,

small 2015, DOI: 10.1002/smll.201500566

Figure 6. a) Contour plot for the calculated induced surface and interface charge density, tot32tot

21totσ ρ σ ρ σ ρ( ) ( ) ( )= + , as a function of ρ ( φ = 360°).

Due to the nanoscale dimensions of the active sensing area ( d eff < 100 nm), the total charge is calculated for a limited area represented by the blue stripe. b) 3D plot of σ tot ( ρ ) with logarithmic length scale. All calculations were done for V tip = 1 V.



Δ φ, between tip and sample, there is an additional charge

contribution to the total charge, ts tipq r C r V eφ( ) ( )= − Δ⎛

⎝⎜⎞⎠⎟ ,

where C ts is the tip–sample capacitance. The tip induced

charges are compensated by applying a tip voltage, V tip = Δ φ/ e .

Concurrently, at V tip = 0 V, the tip induced surface charges are

not zero but are given by Δ φ. For the measured Δ φ of ≈70 mV

using KPFM (Figure 2 a) the tip induced surface charge

corresponds to 1.2 ± 0.3 e for the small NW at V tip = 0 V.

Acceptor-like surface and interface states, negatively charged

if occupied, are known to be present in silicon with a con-

centration of ≈1 × 10 11 cm 2 . [ 40 ] However, surface states are

an inherent part of any chemical sensor and screen external

electric fi elds Therefore, the measured eLOD is even lower

if taking into account surface states. In the electrostatic

simulations surface states and also ambient atmosphere

(humidity) were neglected. This explains the larger effect (by

a factor of ≈6) of a specifi c charge quantity on the simulated

I D compared with the measured I D in the tip gate experiment

(see Figure 4 a).

3. Conclusion

We have introduced a method to measure the electrostatic

LOD of NW FET-based sensors using scanning gate micros-

copy and analytical modeling. The AFM tip is used as a local

gate to emulate the fi eld effect of a molecular gate where ana-

lytes adsorb and induce a surface potential change. We have

demonstrated that the electrostatically formed nanowire-

based chemical sensor can potentially achieve an eLOD of a

single elementary charge.

4. Experimental Section

Scanning Gate Microscopy and Kelvin Probe Force Micros-copy : SGM was carried out with a commercial AFM (Dimension Edge, Bruker) inside a nitrogen glove box with less than 1 ppm H 2 O. Highly conductive cantilevers with Pt/Ir coating (PPP EFM, Nanosensors) were used for both SGM and KPFM. In order to minimize the height error of the AFM tip (<1 nm) due to uncom-pensated electrostatic forces, [ 41–43 ] which increase as a function of V tip , the measurements with a free vibration amplitude, A 0 , of below 15 nm were performed and a relatively low amplitude set point, A sp ≤ 5 nm, for the feedback loop that controls the tip–sample distance was used. [ 43 ] The CPD was measured simultane-ously with the topographic signal using amplitude modulation KPFM at an effective tip sample distance of 5–10 nm during scan-ning. The topographic height was obtained by maintaining the amplitude of the fi rst cantilever resonance ( f 1st ≈ 75 kHz) at a pre-defi ned amplitude set point of ≈5 nm. The CPD was determined by compensating the ac component of the electrostatic force, F ES , at angular frequency ω with an applied dc voltage (= |CPD|) in a feedback control loop. To separate topographic from CPD signal, increase the sensitivity, and minimizing probe convolution effects, the ac electrostatic force component was generated at the second resonance, [ 44,45 ] f 2nd ≈ 450 kHz, of the cantilever by applying an ac voltage of about 500 mV. The CPD of the EFN sensing area was measured before and after exposure to ≈2000 ppm of ethanol vapor in order to demonstrate the molecular gating effect. For this purpose ethanol vapor was introduced inside the glove box while the EFN device was operated and the drain current measured simultaneously.

EFN Device Fabrication : The EFN transistors were fabricated by a semiconductor foundry (TowerJazz, Migdal Haemek, Israel) in a conventional and low-cost CMOS process with four masks to implant the different dopant regions for the channel, source–drain and junction gate contacts. The actual doping densities, blanket Arsenic of 4 × 10 17 cm −3 , junction gate Boron of 2 × 10 20 cm −3 , and source–drain Arsenic of 7 × 10 19 cm −3 , were determined post-fabrication by time-of-fl ight secondary ion mass spectrometry (TOF-SIMS). The measured doping density depth profi les served as input for 3D electrostatic simulations. Boron doped 8 in. SOI wafers with an initial doping density of 1.5 × 10 14 cm 2 and a thick-ness of 150 nm were used. The thickness of the buried SiO 2 was 1.5 µm. The thermal SiO 2 gate dielectric was formed at 1200 °C. The critical dimension (lowest spatial limit) of the process was (540 ± 20) nm, defi ned by the spacing between the two p-doped regions (junctions) in the active area of the sensor. The wafer was diced to 1 cm 2 squares and Ti/Au contacts were manufactured by optical lithography and subsequent metal evaporation.

Electrical Characterization and Gas Sensing : Current–voltage characteristics as a function of drain electrode bias ( I D – V D ), back gate electrode bias ( I D – V BG ), and junction gate electrodes ( I D – V JG12 ), were performed using a semiconductor parameters ana-lyzer (B1500A, Agilent). Sensing of ethanol vapor was done in a

small 2015, DOI: 10.1002/smll.201500566

Figure 7. Drain current plotted as a function of time for different tip voltages and a fi xed tip position above the nanowire-like conductive channel for d eff = 30 nm ( V JG12 = −1.5 V). The graph shows the minimum detectable tip voltage expressed in terms of a charge equivalent (= eLOD) obtained from the analytical calculations. V D = 1 V and V BG = 0 V.

Table 1. Comparison of measured detection limits between electrostatic NWs with two different diameters, d eff of 80 and 30 nm, respectively. S e denotes the electrostatic sensitivity.

d eff [nm]

V tip min [mV]

ΔI D [ e −1 ]

S e [ e −1 ]

LOD Ψ [mV]

eLOD [e]

80 60 2.5 × 10 −10 0.01 0.9 2.3 ± 0.4

30 50 1.25 × 10 −9 0.02 0.5 1.1 ± 0.3


7© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.small-journal.comsmall 2015, DOI: 10.1002/smll.201500566

controlled N 2 (99.999% purity) atmosphere in a sealed metallic gas chamber connected to a gas dilution system. Ethanol gas was generated in a bubbler system and diluted with N 2 and mass fl ow controllers. A reference sensor (ppbRAE 3000, RAE Systems) was connected to the gas chamber in order to monitor the analyte con-centration inside the chamber down to 100 ppb level.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements

A.H. acknowledges the support of the Tel Aviv University Center for Nanoscience and Nanotechnology.

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