Suspended graphene sensors with improved signal and reduced noise

Suspended Graphene Sensors with Improved Signal and Reduced Noise

Zengguang Cheng,Qiang Li,Zhongjun Li,Qiaoyu Zhou,and Ying Fang*

National Center for Nanoscience and Technology,11Beiyitiao Street,Zhongguancun,Beijing 100190,People’s Republic of China

ABSTRACT We report enhanced performance of suspended graphene ?eld effect transistors (Gra-FETs)as sensors in aqueous solutions.Etching of the silicon oxide (SiO 2)substrate underneath graphene was carried out in situ during electrical measurements of devices,which enabled systematic comparison of transport properties for same devices before and after suspension.Signi?cantly,the transconductance of Gra-FETs in the linear operating modes increases 1.5and 2times when the power of low-frequency noise concomitantly decreases 12and 6times for hole and electron carriers,respectively,after suspension of graphene in solution from the SiO 2substrate.Suspended graphene devices were further demonstrated as direct and real-time pH sensors,and complementary pH sensing with the same nanodevice working as either a p-type or n-type transistor was experimentally realized by offsetting the electrolyte gate potential in solution.Our results highlight the importance to quantify fundamental parameters that de?ne resolution of graphene-based bioelectronics and demonstrate that suspended nanodevices represent attractive platforms for chemical and biological sensors.

KEYWORDS Graphene,noise,suspended,pH sensing

G

raphene is attracting tremendous interests due to its nearly perfect crystal quality and superb physical properties,1-5and emerging applications are being

developed based on this novel building block.6-9In particu-lar,owing to its exceptional carrier mobility and one-atomic thickness,graphene ?eld effect transistors (Gra-FETs)have been proposed to hold great potentials for sensitive and label-free detection of chemical/biological species.9To date,studies have been focused exclusively on graphene sensors supported on silicon oxide (SiO 2)substrates,10-13although charge traps at the interface and in the oxide have been shown to act as external scattering centers and degrade transport properties in single-layer graphene whose atoms are all exposed directly to ?uctuations of extrinsic impuri-ties.14-18We have undertaken and report herein studies on performance improvement of graphene devices by suspend-ing them in aqueous solution through a novel in situ etching technique.Our results show that,owing to concomitantly increased transconductance and decreased noise level by

of the oxide,the signal-to-noise ratios of suspended nanodevices were improved by 14dB in low-regime (below 1kHz)for both hole and electron carriers compared with those supported on SiO 2substrates.As an example,suspended devices were demonstrated as real-time and sensitive pH sensors,and complementary detection with either holes or electrons as charge carriers in the same graphene device was achieved,opening up new opportunities for suspended graphene as ?exible candidates for bioelectronics.

Figure 1illustrates the overall design of our experiments.First,Gra-FETs were fabricated from mechanically exfoliated single-layer graphene supported on top of a silicon substrate with 300nm SiO 2.5,19Brie?y,source-drain contacts to graphene were de?ned by e-beam lithography and subse-quently metallization with 5nm Cr/70nm Au.Nanodevices in our study were shown to have smooth graphene surface with height of ～1.0nm by atomic force microscopy (AFM)(see Supporting Information,Figure S1a).Typical 2-probe mobilities of a 5μm long Gra-FET gated through 300nm SiO 2in air were calculated as 4600and 4100cm 2/V-sec for hole and electron carriers (see Supporting Information,Figure S1b),respectively,which are consistent with reported values for pristine graphene at room temperature.5

*To whom correspondence should be addressed.E-mail:fangy@https://www.360docs.net/doc/f4485029.html,.Received for review:02/22/2010Published on Web:00/00/0000

FIGURE 1.Graphene FETs in the electrolyte solution.(a)Schematic representation of our experimental setup where a single-layer graphene is supported in solution by Cr/Au contacts to bridge a trench in the oxide.(b)In situ etching of SiO 2underneath graphene.The conductance of graphene starts to drop gradually after buffered HF was added to the PDMS chamber.Single-layer devices usually stabilize within 50to 100s,indicating the complete suspension of graphene in solution.Arrows indicate the time when solution was switched in the PDMS chamber.The inset shows a scanning electron microscope (SEM)image taken of a suspended graphene device after solution measurements.Scale bar is 0.5μm.

A polydimethylsiloxane (PDMS)chamber was then in-corporated over the Gra-FET chip to con?ne the electrolytic solution of 100mM potassium chloride and 10mM phos-phate (pH7)(Figure 1a).A nonleak Ag/AgCl reference elec-trode was placed in the center of the reservoir as the electrolyte gate.The conductance of graphene sensors was monitored through the source and drain gold electrodes using lock-in detection by applying a 30mV sinusoidal bias voltage (V sd )with a frequency of 109Hz.

To quantify exclusively effects from the underlying oxide

substrate,we developed an in situ etching technique

by carefully keeping devices immersed in liquid during switch-ing of solutions and measuring of Gra-FETs’conductance.We eliminated the drying steps of suspended graphene devices reported by former studies 14,16because the cycle of drying and wetting can easily generate stress to deform graphene and their metal contacts,and alter the electrical properties of devices unintentionally (see Supporting Infor-mation,Figure S2).Etching of SiO 2underneath graphene was monitored in real-time by tracking conductance signals through devices (Figure 1b),and we found that incubation in buffered HF results in ca.100nm/min etching rate underneath graphene.We note that etching of devices in the following discussion was carried out for 1min.14,20,21

The electrical measurements of a typical Gra-FET (0.5μm long and 0.6μm wide)before and after suspension of graphene in solution were summarized in Figure 2and Supporting Information Figure S3.The black curve in Figure 2shows the conductance (d I /d V sd or G )of the device plotted against the electrolyte gate voltage (V gate )with graphene supported on SiO 2substrate.The transconductance of holes (d G /d V gate )in the nanodevice reaches as high as 1mS/V in the linear operating mode because the electrolyte gate was able to modulate effectively the Fermi level in single-layer graphene through the double layer of ions.22Note that the

conductance of the device is large at negative electrolyte gate,and it starts to decrease with increased gate voltage due to the depletion of hole carriers in the p-type graphene until reaches its minimum at the Dirac point,then the conductance starts to increase again with the accumulation of electron carriers in the currently n-type graphene.Inter-estingly,the Dirac point of the graphene device on SiO 2settles at 0.1V when the chemical potential of the neutral solution (pH 7)is at 0V.We observed a large range of Dirac point potentials in our Gra-FETs on SiO 2,(0.1(0.2)V,which was also reported by similar studies on graphene with naked surfaces.12,13,23This variation of device properties in Gra-FETs on SiO 2is apparently disadvantageous in practical applications where reproducible characteristics are critical for devices as,for example,reliable clinical diagnostic sensors.24

After in situ etching of SiO 2,the response of the Gra-FET’s conductance to the electrolyte gate potential was character-ized again in the neutral electrolyte solution as shown by the red curve in Figure 2.Valuable information can be im-mediately deduced by comparing transport properties of the same device before and after suspension.(1)The Dirac point of graphene moved to ～0V after etching of oxide,which was reproduced by other devices in our study,indicating that the Fermi level of suspended graphene is controlled solely by the solution.(2)The operating principle of electrical sensors is based on their conductance changes responding to local potential changes,so their signal sensitivity can be described as d G /d V gate ,that is,the transconductance of devices.10,24Signi?cantly,the transconductance,or the signal sensitivity,of the graphene device in the linear operating regions increases 1.5and 2times for hole and electron carriers,respectively,after suspension.(3)The hole -electron asymmetry was found for both electrolyte-gate and back-gate geometry of the Gra-FET on SiO 2(black curve in Figure 2and Supporting Information Figure S3b).Asymmetric hole and electron branches were previously reported in both nonsuspended and suspended Gra-FETs with channel length below 1μm in vacuum,although the origin of the asymmetry was not understood then.14Surpris-ingly,the asymmetry observed in our short-channel devices decreases greatly after suspension of graphene in electrolyte solutions (red curve in Figure 2),indicating effects from dielectric screening and reduced scattering may be respon-sible for the improved symmetry in solution,25,26although a detailed study is needed in future to reveal the underlying mechanism.

The level of signal that can be processed,or the resolu-tion,of a sensor is ultimately determined by its signal-to-noise ratio.Thus the noise spectra of graphene devices in electrolyte solutions were further examined under our ex-perimental condition,and we will concentrate our discussion on low-frequency (below 1kHz)noise which has been re-ported to be dominant in limiting performance of nanode-vices.15Figure 3a and Supporting Information Figure S4b

FIGURE 2.Transport characterization of a Gra-FET in solution.Black curve is conductance vs the electrolyte gate voltage of the graphene device supported on SiO 2substrate,and red curve is of the same device after suspension of graphene in solution.The black and red diamonds highlight electrolyte gate potentials at which the com-parison of transconductance and noise spectra was performed in the text and Figure 3.

summarize the current noise power spectral density normal-ized by the mean square of current,S)(S I)/(I2),as a function of frequency,f,of the same graphene device in Figure2 before suspension.The normalized power spectra overlap in the linear operating modes and around the Dirac point of the Gra-FET,indicating that noise density in graphene gated by the electrolyte gate is independent of the gate potential, although gate-dependent noise spectra have been reported in graphene nanobelts gated through SiO2dielectric.15Fur-ther,the power spectral density of the graphene device was inversely proportional to the frequency,representing the1/f ?icker noise characteristic ubiquitous in nanodevices.Thus the noise spectral density in graphene devices supported on SiO2can be expressed as27

in the low-frequency region for the electrolyte gate geometry in solution,where A～10-6is the amplitude of the1/f noise,and S I has a unit of A2/Hz.Thus the power of the current noise,P noise,for a given frequency band[f1,f2]in the low-frequency region can be calculated as

and P noise is with a unit of A2.

After suspension of graphene in solution,the normalized power spectral density of the device dropped12and6times for hole and electron carriers,respectively,compared with that on SiO2substrate(Figure3b,c).It has been found that trapped charges at the interface and in the oxide degrade transport characteristics of single-layer graphene.14,17Former study of graphene devices on SiO2substrate with low 2-probe carrier mobility in vacuum(～100cm2/V-sec)were reported to show～20times improved performance by ion screening of external impurity charges in high ionic solu-tions.25Our Gra-FETs,on the other hand,have high mobility (4000to5000cm2/V-sec)on SiO2in the back-gate geometry

FIGURE3.Noise characterization of graphene device in electrolyte solution.(a)Normalized current power spectral density is shown as a function of frequency of the graphene device on SiO2at different electrolyte gate potentials.The power spectra were measured at a DC mode with a bias voltage of30mV,and sharp peaks(50Hz and its harmonics)were caused by extrinsic noise from the power source.(b)Comparison of graphene’s noise power spectra in the linear operating modes with holes as carriers before(black,V gate)-0.10V)and after suspension (red,V gate)-0.15V).(c)Comparison of graphene’s noise power spectra with electrons as carriers before(black,V gate)0.20V)and after suspension(red,V gate)0.15V).

S I)AI2

f

P noise)∫f

1

f2

S I d f)∫f1f2AI2f d f)AI2ln f2f

1

in air.The further improved performance in our suspended graphene shows that device degradation associated with oxide substrates is still dominant in our nonsuspended devices,despite any possible ionic screening effect in100 mM solutions.It is important to note that low ionic strength that offers long Debye length is desirable for detection of charged biomolecules with nano-FETs,24,28thus the elimina-tion of the oxide by suspension can be increasingly critical to the reduction of noise for single-layer graphene in solu-tions with low ionic strength.

Overall,the signal-to-noise ratio de?ned as29

was increased by14dB in low-frequency regime for both hole and electron carriers in graphene devices after suspen-sion.Thus our results explicitly prove that for single-layer graphene FET based sensors,a platform with suspended nanodevices is advantageous to achieve low detection limit in solution.

To demonstrate the viability of suspended graphene device as direct chemical sensors,we applied our nanode-vices to pH sensing.Solutions from pH6to pH9were delivered to the surface of a graphene device sequentially, and for each solution,the electrolyte gate response of the device’s conductance was measured as shown in Figure4a. We found that the Dirac point of graphene shifts positively from pH6to pH9(see Supporting Information,Figure S5), which is consistent with the increased chemical potential of solution and con?rms the capability of suspended graphene for chemical sensing in solution.30-32

Compared to former unipolar silicon nanowire(SiNW)or carbon nanotube(CNT)based p-type sensors,33,34suspended graphene shows nearly symmetric transport around the Dirac point in neutral solution(Figure4a),so it can work as either a p-type or n-type sensor when a small gate potential,without introducing any redox reaction of molecules in solution,is applied to switch its carrier characteristic.We investigated the time-dependent response of the Gra-FET’s conductance to different pH solutions when the electrolyte gate is biased at -0.05and0.05V,respectively.The conductance of the nano-sensor was monitored in real-time as solutions with pH6to pH9were sequentially delivered to the surface of the sus-pended graphene.As shown in Figure4b,the graphene works as a p-type material when-0.05V gate potential is applied. Similar to former p-type devices of SiNWs or CNTs,33,35con-ductance of the graphene device increases with increased pH values.On the contrary,when the gate potential is switched to +0.05V,the signal?ips its sign and the conductance of the graphene device decreases with increased pH.The polarity?ip of signals veri?es that the sensing mechanism in graphene nanodevices is due to a?eld-effect.The unique capability to tune easily Gra-FETs between p-type and n-type characteristics presents powerful con?rmation on the nature of sensing sig-nals.We further note that the pH detection of our graphene devices is reversible.Thus complementary and reversible measurements in different operating regions of graphene de-monstrate the?exibility of our suspended graphene sensors.

In summary,quantitative studies on the improved per-formance of suspended graphene devices in solution were reported.Our results show that,in low-frequency regime, the signal-to-noise ratio of graphene devices increased14 dB for both hole and electron carriers as a result of concomi-tantly increased mobility and decreased noise level by suspension in solution.Direct and real-time detection of solution’s pH by suspended graphene was described,and complementary signals from the same graphene device as a p-type or n-type transistor were presented.The enhanced electrical characteristics of suspended graphene devices in aqueous solution show that they are advantageous for chemical and biological detection than their counterparts supported on substrates,and our results are expected to

SNR(dB))20log A signal

A noise

)10log

P signal

P noise

FIGURE4.Graphene as pH sensor.(a)Gra-FET’s conductance as a

function of the electrolyte gate voltage in pH6,7,8,and9solutions.

(b)Real-time detection of solution’s pH by conductance change of

the same graphene device with the electrolyte gate potential biased

at-0.05(left branch)and+0.05V(right

branch).

bring attentions to suspended nanodevices in general as promising platforms for bioelectronics.

Acknowledgment.Y.F.acknowledges support of this work by Special Presidential Foundation of the Chinese Academy of Sciences,China (08172911ZX),“973”Fund (2009CB930200),and the National Natural Science Founda-tion of China (20973045).

Supporting Information Available.Methods and Figures S1-S5.This material is available free of charge via the Internet at https://www.360docs.net/doc/f4485029.html,.

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