- Email: [email protected]
Enhanced electrical performance and stability of graphene- l -cysteine bioelectronic devices
Accepted Manuscript Enhanced electrical performance and stability of Graphene- L-Cysteine bioelectronic devices Salma Siddique, Muhammad Zahir Iqbal, Hamid Mukhtar PII:
S0254-0584(17)30682-X
DOI:
10.1016/j.matchemphys.2017.08.059
Reference:
MAC 19957
To appear in:
Materials Chemistry and Physics
Please cite this article as: Salma Siddique, Muhammad Zahir Iqbal, Hamid Mukhtar, Enhanced electrical performance and stability of Graphene- L-Cysteine bioelectronic devices, Materials Chemistry and Physics (2017), doi: 10.1016/j.matchemphys.2017.08.059 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Enhanced Electrical Performance and Stability of Graphene- L-Cysteine Bioelectronic Devices
Institute of Industrial Biotechnology, GC University Lahore, Lahore 54000, Pakistan
2
Faculty of Engineering Sciences, GIK Institute of Engineering Sciences and Technology, Topi
23640, Khyber Pakhtunkhwa, Pakistan
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*Email: [email protected] ; Phone:+92 348 5968405
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1
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Salma Siddique1, Muhammad Zahir Iqbal2,*, Hamid Mukhtar1
Abstract
The study of interaction between biomolecule and graphene is of great interest for future bioelectronic applications. Here, we report the adsorption properties of L-cysteine biomolecules
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on the surface of graphene sheet. Raman spectroscopy was used to characterize the intrinsic properties of graphene. We further performed the electrical transport measurement to investigate
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the effect of L-cysteine biomolecules on the graphene field effect transistors (FETs). We found that the charge neutrality point (CNP) is shifting towards negative gate voltage due to
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biochemical dopant which demonstrate n-type doping in the graphene film and the effect become prominent by increasing the chemical treatment time. The electron mobility of the graphene FET is drastically enhanced up to five times while the hole mobility is increased approximately twice as a function of treatment time. Therefore, the elevated values of mobility signifies the improvement in performance of the L-cysteine coated graphene based FETs. Thus, biomolecular treatment of graphene could be an effective approach to expedite for bioelectronic and biosensing devices.
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Keywords: Graphene, L-cysteine, FET, n-type doping, electrical properties, stability.
1. Introduction
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The high surface sensitivity of graphene offers an innovative direction towards variety of applications, such as sensing, monitoring, and detection of different molecules [1-5]. Another aspect of responsive nature of graphene surface is to modulate the graphene properties itself with the adsorption of different molecules. Several approaches are being employed to modulate the
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graphene’s properties using chemical doping [6-14]. However, most of the chemical treatment
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cause structural disorders in graphene which leads to the mobility degradation and certainly reduce the performance of device. According to previous reports, graphene surface become structurally disordered when it is treated with tetrasodium 1,3,6,8-pyrenetetrasulfonic acid (TPA), Tetracyanoethylene (TCNE)-EG, 2,3-dimethoxy-1,3-butadiene (DMBD), aromatic diazonium ions, 2,3-dimethoxy-1,3-butadiene (DMBD) and maleic anhydride epitaxial graphene
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(MA)-EG [15, 16]. Therefore, it is highly desirable to determine an ideal classification of chemicals or any biomolecules that could improve the properties of graphene without any structural disorder. The interaction mechanism of biomolecules on the surface of graphene is
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challenging because of their complex structures. However, the interaction of small molecules can
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be easily investigated such as amino acids have small polypeptides building blocks structure. Some of the experimental and theoretical studies have reported the interaction mechanism of amino acids such as L-cysteine by density functional theory (DFT) calculations [17-26]. Lcysteine (HS-CH2-CH-(NH2)-COOH) structure contains three functional groups i.e. (amino (NH2), carboxyl (-COOH) and thiol (-SH) group) head groups at the different binding sites, which are responsible for interaction with the neighboring inorganic environment.
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In this work, we investigate the modulation in graphene’s properties after surface treatment with L-cysteine molecules. Raman spectroscopy is utilized to analyze the intrinsic properties of graphene as well as the effect of immobilized molecules. We further investigated the effect of
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biomolecular doping by electrical transport measurement for different periods of treatment time. The chemical vapor deposition (CVD) grown graphene is used in this work because of comparatively low cost, easily transferable to desired substrate and promising technique for
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large-scale production. Our results have highlighted the importance of L-cysteine molecule, the
graphene from any structural disorder.
2. Experimental
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treatment of which not only improves the intrinsic properties of graphene but also prevents
2.1. Graphene growth and transfer process
The graphene samples were grown over the large area by CVD method on 25 µm thick copper
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(Cu) foil (Alpha Aesar (99.8% pure)). An electromechanical polished Cu foil was placed in the tube furnace. Initially, the H2 gas was purged in the tube with pressure of ~10-2 Torr. In the
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meantime temperature was gradually increased up to 1010 ̊C. Next, the furnace pressure was maintained up to ~10-4 Torr pressure. The mixture of CH4 (20 sccm) and H2 (5 sccm) was then
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introduced into the furnace tube for 8 minutes to grow graphene by maintaining temperature up to 1010 ̊C [27, 28]. Afterward, temperature of furnace was lower down gradually to room temperature at the rate of 50 ̊C/min. The obtained graphene film of Cu foil was then transferred on silicon dioxide supported with p-doped silicon substrate by following process. A thin layer of polymethyl methacrylate (PMMA) was spin coated (850 rpm for 10 sec, 2500 rpm for 60 sec) on to the graphene surface and heated the sample at 180 ̊C for 3 min. Then the bottom Cu foil of graphene was dissolved by etching in 1M solution of ammonium persulfate (APS, (NH4)2S2O8).
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After washing with deionized water, the PMMA coated graphene film was then transferred on to the SiO2/Si substrate. PMMA layer was removed by placing Graphene/SiO2/Si sample in acetone
2.2. L-cysteine doping and characterizations
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for 6 hours. Then, the methanol was used to rinse and dried with nitrogen gas.
Cysteine (α-amino acid (C3H7NO2S)) is highly soluble compound in water with solubility 280
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g/L at 25 ̊C and having molecular weight of 121.16 g/mol. The modifications of L-cysteine treated graphene devices were studied by Raman spectroscopy and transport measurements. The
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measurements were performed for pristine and different treatment times (1, 5, 15, 30, 45, 60 min) with 1 molar cysteine solution. Renishaw Raman spectrometer with wavelength 514 nm was utilized over a wave number from 1100 cm-1 to 3200 cm-1 to study the intrinsic properties as well as the doping tunability in graphene. The electrical properties of graphene FETs (pristine
vacuum conditions.
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and after various treatments) were investigated with four-probe transport measurements setup in
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3. Results and Discussion
Figure 1(a) shows the schematic of L-cysteine immobilized graphene based FET device. The
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graphene sheet is transferred onto the Si substrate containing top layer of 300 nm thick SiO2. Figure 1(b) represents the optical image of graphene based FET. Figure 1(c) illustrates the chemical structure of cysteine, while ball and stick structure of cysteine is shown in Fig. 1(d). Figure 2(a) shows the Raman spectra of monolayer graphene before and after treatment with cysteine over a period of different reaction time. Raman spectroscopy is a commonly used nondestructive tool to examine the structural properties of graphene. The characteristic G and 2D Raman bands can confirm the number of stacked graphene layers and the shift in the peak
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positions determine the modulation in charge carrier concentration [29-32]. The characteristic G and 2D peaks appear at 1589 cm-1 and 2689.5 cm-1, respectively showing the similar pattern as reported earlier [33]. The G peak is shifted from 1589 cm-1 to 1584.9 cm-1 and 2D peak shifted
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from 2689.5 cm-1 to 2684.5 cm-1. This downward shift of the G and 2D peak positions is attributed to the n-type doping as reported earlier for the doped graphene samples [34]. Figure 2 (b) shows the overall trend of G and 2D peaks with increasing treatment time. Figure 2(c)
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illustrates the intensity of I2D/IG and ID/IG ratio as a function of treatment time. The I2D/IG ratio is 3.47 for pristine graphene which determines the single layer of graphene and the ratio is
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increased with increasing reaction time, which indicates that the quality of graphene is improved after functionalization with cysteine. However, there is negligible variation observed in ID/IG which indicates that the cysteine molecule do not cause any structural defects in the lattice of graphene layer.
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Figure 3(a) shows the resistivity versus gate voltage (Vg) for pristine and after treatment of graphene in 1 M cysteine solution for different periods of time. The CNP or Dirac point (VDirac) of the pristine graphene is observed at Vg = +64 V. The perfect graphene with no impurities
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should show an infinite resistance at the CNP. However, experiment results reveal a finite resistance at Dirac point because of the existence of electron hole puddles [35, 36]. After
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treatment of graphene with cysteine solution for 1 min, the VDirac shifted from +64 V to +15V, which is attributed to the addition of electrons in the graphene. This VDirac shift increases towards negative gate voltage when treatment time was enlarged and it reached up to -45 V after 60 min treatment. The huge shift towards negative gate voltage indicates the strong n-type doping effect in graphene device. The Dirac point shift become saturated when the graphene surface become completely absorbed for a certain time period. The VDirac shift of graphene device as a function
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of reaction time in cysteine solution is shown in Fig. 3 (b), while the inset shows the I-V characteristic curves at Vg= 0 for pristine graphene and after different treatments. These curves demonstrate a systematic trend which are tuned with the doping level. The carrier density of
from the given equation, =
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graphene is calculated at different back gate voltages as shown in Fig. 3(c), which is calculated ( − ), where ‘Cg’ is the gate capacitance of 300 nm thick
silicone oxide layer having value of ∼115 aF/μm2 [37] , ‘VD’ is the Dirac point of graphene, ‘Vg’
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is the back gate voltage and ‘e’ is the electronic charge. The carrier density ‘n’ of graphene is
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associated to the change in Fermi level of graphene layer by L-cysteine-doping. The field effect mobility of the samples before and after treatment were obtained by using this formula, μ =
(
), where σ = 1/ ρ is the conductivity of samples [38]. The factor (
) is representing the
slope of the respective conductivity data and is calculated on the basis of line fitted to the linear
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region. The trend of electron and hole mobilities is shown in Fig. 3 (d). The hole mobility increases from 1390 to 2449 cm2/Vs after 60 min, while electron mobility increases gradually from 564 to 2457 cm2/Vs. Figure 4 (a) illustrates the doping stability of finally treated (60 min)
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device and then measured after 15, 30, and 45 days, respectively. The Dirac point is almost in the same position after several days indicating the stability of doping. Electron and hole motilities
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also remained unchanged with the passage of time as shown in Fig. 4 (b). Therefore, doping characteristics and performance of graphene devices can be tuned and sustained for long period of time.
4. Conclusion
We have investigated the doping effect of L-cysteine treated graphene devices for different reaction times. Raman spectroscopy revealed the intrinsic characteristics of pristine and treated samples of graphene devices. The shift of G and 2D peaks towards lower wave number attributes
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the n-type doping without affecting its intrinsic structure. The resistivity as a function of gate voltage also demonstrates the gradual increase of n-type doping in graphene FET with increasing treatment time. The hole and electron mobilities are found to increase from 1390 to 2449 cm2/Vs
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and 564 to 2457 cm2/Vs, respectively. The increasing mobilities trend demonstrates the overall improvement in the performance of graphene devices. Furthermore, the doping effect is found to be stable and reproducible with the passage of time without degradation of mobility. The
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progressive enhancement in the mobilities with reaction time and the long time stability of the graphene FET devices indicate that chemical treatment is a convenient approach to tailor the
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electrical properties. L-cysteine treatment reflects its compatibility with graphene which could be utilized for future bioelectronic applications.
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Acknowledgements
This work is supported by Higher Education Commission (HEC) of Pakistan under the National
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Research Program for Universities (NRPU) with Project No. MS5544.
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Figure captions
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Fig. 1 (a) Schematic view of L-cysteine immobilized graphene FET on the SiO2/Si substrate. (b) Optical image of CVD grown graphene FET on the SiO2/Si substrate with top Au electrodes. (c) 2D representation of cysteine molecule. (d) Ball and stick 3D structure of cysteine.
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Fig. 2 (a) Raman spectra of pristine and cysteine modified graphene. (b) Zoomed image of G and
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2D peaks of pristine and cysteine modified graphene. (c) The trend of shift in G and 2D peak positions with cysteine treatment for different periods of time.
Fig. 3 (a) Resistivity versus back gate voltage (Vg) for graphene before and after cysteine treatment for different period of time. (b) Shift of Dirac point positions as function of reaction time in cysteine solution. (c) Charge carrier concentration at different back gate voltage as a
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function of reaction time. (d) Electron and hole field mobility as a function of reaction time. Fig. 4 (a) Doping stability results after 15, 30 and 45 days, respectively. (b) Hole and electron
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motilities sustainability with the passage of time.
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Figure 4
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•
Effect of L-cysteine interaction on the graphene surface is investigated. Surface modifications induce n-type doping in the graphene FETs. L-cysteine treatment on graphene surface improve the charge carrier mobility and electrical performance of FET devices. Doping effect is found to be stable and reproducible with the passage of time without degradation of mobility.
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• • •
S0254-0584(17)30682-X
DOI:
10.1016/j.matchemphys.2017.08.059
Reference:
MAC 19957
To appear in:
Materials Chemistry and Physics
Please cite this article as: Salma Siddique, Muhammad Zahir Iqbal, Hamid Mukhtar, Enhanced electrical performance and stability of Graphene- L-Cysteine bioelectronic devices, Materials Chemistry and Physics (2017), doi: 10.1016/j.matchemphys.2017.08.059 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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M AN U
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ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT
Enhanced Electrical Performance and Stability of Graphene- L-Cysteine Bioelectronic Devices
Institute of Industrial Biotechnology, GC University Lahore, Lahore 54000, Pakistan
2
Faculty of Engineering Sciences, GIK Institute of Engineering Sciences and Technology, Topi
23640, Khyber Pakhtunkhwa, Pakistan
M AN U
*Email: [email protected] ; Phone:+92 348 5968405
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1
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Salma Siddique1, Muhammad Zahir Iqbal2,*, Hamid Mukhtar1
Abstract
The study of interaction between biomolecule and graphene is of great interest for future bioelectronic applications. Here, we report the adsorption properties of L-cysteine biomolecules
TE D
on the surface of graphene sheet. Raman spectroscopy was used to characterize the intrinsic properties of graphene. We further performed the electrical transport measurement to investigate
EP
the effect of L-cysteine biomolecules on the graphene field effect transistors (FETs). We found that the charge neutrality point (CNP) is shifting towards negative gate voltage due to
AC C
biochemical dopant which demonstrate n-type doping in the graphene film and the effect become prominent by increasing the chemical treatment time. The electron mobility of the graphene FET is drastically enhanced up to five times while the hole mobility is increased approximately twice as a function of treatment time. Therefore, the elevated values of mobility signifies the improvement in performance of the L-cysteine coated graphene based FETs. Thus, biomolecular treatment of graphene could be an effective approach to expedite for bioelectronic and biosensing devices.
ACCEPTED MANUSCRIPT
Keywords: Graphene, L-cysteine, FET, n-type doping, electrical properties, stability.
1. Introduction
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The high surface sensitivity of graphene offers an innovative direction towards variety of applications, such as sensing, monitoring, and detection of different molecules [1-5]. Another aspect of responsive nature of graphene surface is to modulate the graphene properties itself with the adsorption of different molecules. Several approaches are being employed to modulate the
SC
graphene’s properties using chemical doping [6-14]. However, most of the chemical treatment
M AN U
cause structural disorders in graphene which leads to the mobility degradation and certainly reduce the performance of device. According to previous reports, graphene surface become structurally disordered when it is treated with tetrasodium 1,3,6,8-pyrenetetrasulfonic acid (TPA), Tetracyanoethylene (TCNE)-EG, 2,3-dimethoxy-1,3-butadiene (DMBD), aromatic diazonium ions, 2,3-dimethoxy-1,3-butadiene (DMBD) and maleic anhydride epitaxial graphene
TE D
(MA)-EG [15, 16]. Therefore, it is highly desirable to determine an ideal classification of chemicals or any biomolecules that could improve the properties of graphene without any structural disorder. The interaction mechanism of biomolecules on the surface of graphene is
EP
challenging because of their complex structures. However, the interaction of small molecules can
AC C
be easily investigated such as amino acids have small polypeptides building blocks structure. Some of the experimental and theoretical studies have reported the interaction mechanism of amino acids such as L-cysteine by density functional theory (DFT) calculations [17-26]. Lcysteine (HS-CH2-CH-(NH2)-COOH) structure contains three functional groups i.e. (amino (NH2), carboxyl (-COOH) and thiol (-SH) group) head groups at the different binding sites, which are responsible for interaction with the neighboring inorganic environment.
ACCEPTED MANUSCRIPT
In this work, we investigate the modulation in graphene’s properties after surface treatment with L-cysteine molecules. Raman spectroscopy is utilized to analyze the intrinsic properties of graphene as well as the effect of immobilized molecules. We further investigated the effect of
RI PT
biomolecular doping by electrical transport measurement for different periods of treatment time. The chemical vapor deposition (CVD) grown graphene is used in this work because of comparatively low cost, easily transferable to desired substrate and promising technique for
SC
large-scale production. Our results have highlighted the importance of L-cysteine molecule, the
graphene from any structural disorder.
2. Experimental
M AN U
treatment of which not only improves the intrinsic properties of graphene but also prevents
2.1. Graphene growth and transfer process
The graphene samples were grown over the large area by CVD method on 25 µm thick copper
TE D
(Cu) foil (Alpha Aesar (99.8% pure)). An electromechanical polished Cu foil was placed in the tube furnace. Initially, the H2 gas was purged in the tube with pressure of ~10-2 Torr. In the
EP
meantime temperature was gradually increased up to 1010 ̊C. Next, the furnace pressure was maintained up to ~10-4 Torr pressure. The mixture of CH4 (20 sccm) and H2 (5 sccm) was then
AC C
introduced into the furnace tube for 8 minutes to grow graphene by maintaining temperature up to 1010 ̊C [27, 28]. Afterward, temperature of furnace was lower down gradually to room temperature at the rate of 50 ̊C/min. The obtained graphene film of Cu foil was then transferred on silicon dioxide supported with p-doped silicon substrate by following process. A thin layer of polymethyl methacrylate (PMMA) was spin coated (850 rpm for 10 sec, 2500 rpm for 60 sec) on to the graphene surface and heated the sample at 180 ̊C for 3 min. Then the bottom Cu foil of graphene was dissolved by etching in 1M solution of ammonium persulfate (APS, (NH4)2S2O8).
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After washing with deionized water, the PMMA coated graphene film was then transferred on to the SiO2/Si substrate. PMMA layer was removed by placing Graphene/SiO2/Si sample in acetone
2.2. L-cysteine doping and characterizations
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for 6 hours. Then, the methanol was used to rinse and dried with nitrogen gas.
Cysteine (α-amino acid (C3H7NO2S)) is highly soluble compound in water with solubility 280
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g/L at 25 ̊C and having molecular weight of 121.16 g/mol. The modifications of L-cysteine treated graphene devices were studied by Raman spectroscopy and transport measurements. The
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measurements were performed for pristine and different treatment times (1, 5, 15, 30, 45, 60 min) with 1 molar cysteine solution. Renishaw Raman spectrometer with wavelength 514 nm was utilized over a wave number from 1100 cm-1 to 3200 cm-1 to study the intrinsic properties as well as the doping tunability in graphene. The electrical properties of graphene FETs (pristine
vacuum conditions.
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and after various treatments) were investigated with four-probe transport measurements setup in
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3. Results and Discussion
Figure 1(a) shows the schematic of L-cysteine immobilized graphene based FET device. The
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graphene sheet is transferred onto the Si substrate containing top layer of 300 nm thick SiO2. Figure 1(b) represents the optical image of graphene based FET. Figure 1(c) illustrates the chemical structure of cysteine, while ball and stick structure of cysteine is shown in Fig. 1(d). Figure 2(a) shows the Raman spectra of monolayer graphene before and after treatment with cysteine over a period of different reaction time. Raman spectroscopy is a commonly used nondestructive tool to examine the structural properties of graphene. The characteristic G and 2D Raman bands can confirm the number of stacked graphene layers and the shift in the peak
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positions determine the modulation in charge carrier concentration [29-32]. The characteristic G and 2D peaks appear at 1589 cm-1 and 2689.5 cm-1, respectively showing the similar pattern as reported earlier [33]. The G peak is shifted from 1589 cm-1 to 1584.9 cm-1 and 2D peak shifted
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from 2689.5 cm-1 to 2684.5 cm-1. This downward shift of the G and 2D peak positions is attributed to the n-type doping as reported earlier for the doped graphene samples [34]. Figure 2 (b) shows the overall trend of G and 2D peaks with increasing treatment time. Figure 2(c)
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illustrates the intensity of I2D/IG and ID/IG ratio as a function of treatment time. The I2D/IG ratio is 3.47 for pristine graphene which determines the single layer of graphene and the ratio is
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increased with increasing reaction time, which indicates that the quality of graphene is improved after functionalization with cysteine. However, there is negligible variation observed in ID/IG which indicates that the cysteine molecule do not cause any structural defects in the lattice of graphene layer.
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Figure 3(a) shows the resistivity versus gate voltage (Vg) for pristine and after treatment of graphene in 1 M cysteine solution for different periods of time. The CNP or Dirac point (VDirac) of the pristine graphene is observed at Vg = +64 V. The perfect graphene with no impurities
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should show an infinite resistance at the CNP. However, experiment results reveal a finite resistance at Dirac point because of the existence of electron hole puddles [35, 36]. After
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treatment of graphene with cysteine solution for 1 min, the VDirac shifted from +64 V to +15V, which is attributed to the addition of electrons in the graphene. This VDirac shift increases towards negative gate voltage when treatment time was enlarged and it reached up to -45 V after 60 min treatment. The huge shift towards negative gate voltage indicates the strong n-type doping effect in graphene device. The Dirac point shift become saturated when the graphene surface become completely absorbed for a certain time period. The VDirac shift of graphene device as a function
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of reaction time in cysteine solution is shown in Fig. 3 (b), while the inset shows the I-V characteristic curves at Vg= 0 for pristine graphene and after different treatments. These curves demonstrate a systematic trend which are tuned with the doping level. The carrier density of
from the given equation, =
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graphene is calculated at different back gate voltages as shown in Fig. 3(c), which is calculated ( − ), where ‘Cg’ is the gate capacitance of 300 nm thick
silicone oxide layer having value of ∼115 aF/μm2 [37] , ‘VD’ is the Dirac point of graphene, ‘Vg’
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is the back gate voltage and ‘e’ is the electronic charge. The carrier density ‘n’ of graphene is
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associated to the change in Fermi level of graphene layer by L-cysteine-doping. The field effect mobility of the samples before and after treatment were obtained by using this formula, μ =
(
), where σ = 1/ ρ is the conductivity of samples [38]. The factor (
) is representing the
slope of the respective conductivity data and is calculated on the basis of line fitted to the linear
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region. The trend of electron and hole mobilities is shown in Fig. 3 (d). The hole mobility increases from 1390 to 2449 cm2/Vs after 60 min, while electron mobility increases gradually from 564 to 2457 cm2/Vs. Figure 4 (a) illustrates the doping stability of finally treated (60 min)
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device and then measured after 15, 30, and 45 days, respectively. The Dirac point is almost in the same position after several days indicating the stability of doping. Electron and hole motilities
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also remained unchanged with the passage of time as shown in Fig. 4 (b). Therefore, doping characteristics and performance of graphene devices can be tuned and sustained for long period of time.
4. Conclusion
We have investigated the doping effect of L-cysteine treated graphene devices for different reaction times. Raman spectroscopy revealed the intrinsic characteristics of pristine and treated samples of graphene devices. The shift of G and 2D peaks towards lower wave number attributes
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the n-type doping without affecting its intrinsic structure. The resistivity as a function of gate voltage also demonstrates the gradual increase of n-type doping in graphene FET with increasing treatment time. The hole and electron mobilities are found to increase from 1390 to 2449 cm2/Vs
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and 564 to 2457 cm2/Vs, respectively. The increasing mobilities trend demonstrates the overall improvement in the performance of graphene devices. Furthermore, the doping effect is found to be stable and reproducible with the passage of time without degradation of mobility. The
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progressive enhancement in the mobilities with reaction time and the long time stability of the graphene FET devices indicate that chemical treatment is a convenient approach to tailor the
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electrical properties. L-cysteine treatment reflects its compatibility with graphene which could be utilized for future bioelectronic applications.
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Acknowledgements
This work is supported by Higher Education Commission (HEC) of Pakistan under the National
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Figure captions
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Fig. 1 (a) Schematic view of L-cysteine immobilized graphene FET on the SiO2/Si substrate. (b) Optical image of CVD grown graphene FET on the SiO2/Si substrate with top Au electrodes. (c) 2D representation of cysteine molecule. (d) Ball and stick 3D structure of cysteine.
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Fig. 2 (a) Raman spectra of pristine and cysteine modified graphene. (b) Zoomed image of G and
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2D peaks of pristine and cysteine modified graphene. (c) The trend of shift in G and 2D peak positions with cysteine treatment for different periods of time.
Fig. 3 (a) Resistivity versus back gate voltage (Vg) for graphene before and after cysteine treatment for different period of time. (b) Shift of Dirac point positions as function of reaction time in cysteine solution. (c) Charge carrier concentration at different back gate voltage as a
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function of reaction time. (d) Electron and hole field mobility as a function of reaction time. Fig. 4 (a) Doping stability results after 15, 30 and 45 days, respectively. (b) Hole and electron
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motilities sustainability with the passage of time.
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Effect of L-cysteine interaction on the graphene surface is investigated. Surface modifications induce n-type doping in the graphene FETs. L-cysteine treatment on graphene surface improve the charge carrier mobility and electrical performance of FET devices. Doping effect is found to be stable and reproducible with the passage of time without degradation of mobility.
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• • •