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Tunable Fermi level of graphene modified by azobenzene molecules
Applied Surface Science 463 (2019) 900–906
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Full Length Article
Tunable Fermi level of graphene modified by azobenzene molecules a,b,1
Jiaojiao Yu , Mingjia Zhang ⁎ Changshui Huanga,
a,1
a
c
d
T a
, Jianjiang He , Chunfang Zhang , Weiwei Cui , Ning Wang ,
a
Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, No. 189 Songling Road, Qingdao 266101, PR China University of Chinese Academy of Sciences, Beijing 100190, PR China Department of Chemistry and Environmental Science, Hebei University, Baoding 071002, PR China d Department of Physics, Qingdao University, Qingdao 266071, PR China b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Graphene Raman spectroscopy Field-effect transistor Fermi level Carrier concentration
Carbon-based nanomaterials, especially graphene, are considered to be the most hopeful alternatives to silicon in the near future of pushing to its limits. However, the zero band gap of graphene suppresses its application, it needs to develop a convenient method to modulate its Fermi level and band gap. Herein, we report a general approach to regulate Fermi level of graphene precisely by designed azobenzene molecules with different dipole moment and dipole orientation. The Raman spectra results demonstrate the π-π interaction between the azobenzene molecules and graphene, which result in the modulation of the Fermi level of graphene. Besides, based on the field effect transistor characteristic measurement, we also observe the Fermi level adjustment for the hole/electron doping to graphene from the spontaneous polarization effect of the azobenzene molecules, further speculating the regulation of electronic structure and providing a new route for changing the electrical properties of graphene.
1. Introduction
Graphene has four half-filled degenerate states at the intrinsic Fermi level which consisting of two degenerate states at two nonequivalent Dirac points (K and K’), comes from the crystal symmetry of graphene's honeycomb lattice. Band gap opening in graphene thus implies breaking of the symmetry [13]. Hence, many efforts have been dedicated to open the band gap of graphene by modulating the Dirac point or Fermi level [14]. It is reported that the graphene Fermi level is modulated by electrical field effect which would result in hole or electron doping [15]. Graphene nanoribbons and atom-doped graphene are also investigated for the application in field effect transistors which demonstrate high on-off ratios [16]. Just recently, the molecules (including gas, molecular acceptor, self-assembled monolayers) adsorbed on graphene are considered to be an effective way to tune the electronic structure of intrinsic graphene because of its harmlessness, simpleness and effectivity [17–20]. However, general rules for the adjustment of graphene Fermi level by adsorbed molecules have not yet been established. Herein, we modify the surface of graphene with the azobenzene molecules with different dipole moments and dipole orientations to realize the n- and p- doping and regulate its Fermi level. Raman spectrum and field effect transistor (FET) are used to characterize the
The foundation of modern information technology is the integrated circuit chip, 90% of which is silicon-based complementary metal-oxidesemiconductor (CMOS) technology. Silicon-based CMOS technology is about to enter the 14 nm node after half a century rapid development and will reach its performance limit [1]. With the increasingly critical research in the post-Moore era of nanoelectronics science, many researchers count on the development of non-silicon-based electronics and abandon the use of silicon as a supporting material to be up against the 8 nm technology node. Among the few possible candidate materials [1–3], carbon-based nanomaterials, especially graphene [4,5], are considered to be the most hopeful alternatives [6] to silicon due to its extremely small thickness and extraordinary carrier mobility [7] and will likely profoundly affect the future development of chips and related industries. Since the discovery of graphene in 2004 [8], it becomes a fascinating material [5] applying to the field of high-frequency transistors, sensors, flexible electronics for its great mechanical strength [9], excellent chemical and thermal stability [10], and brilliant flexibility [11,12]. Nevertheless, the zero band gap severely limits its application in the semiconductor field.
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Corresponding author. E-mail address: [email protected] (C. Huang). 1 These authors contributed equally. https://doi.org/10.1016/j.apsusc.2018.09.021 Received 10 February 2018; Received in revised form 30 August 2018; Accepted 3 September 2018 Available online 04 September 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved.
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J. Yu et al.
surface was cleaned by UV/ozone treatment to remove the contamination from the surface of the wafer and graphene. The photoresist (AR-P 3740) was spin-coated on the surface of graphene at high-speed (1500 rpm) for 30 s to result in a uniform thin layer between 2 and 3 µm thick. Preparation of the resist is concluded prebaking over a hot plate at 100 °C for 30 s to evaporate the photoresist solvent. After prebaking, the sample coated photoresist was exposed to a pattern of intense ultraviolet light within the intensity of 100 mJ/cm2 through photolithographic masks. The samples were developed in developer for 30 s and rinsed using deionized water. The sample was then post-baked to drive off excess water at 120 °C for 60 s on a hotplate. Silver (80 nm) electrode was deposited on the graphene by evaporation. And lift-off process was performed in toluene with immersion times typically of several hours (see Supporting Information Fig. S2).
interaction between the azobenzene molecules and graphene and the modulation in the Fermi level of graphene. The results demonstrate that the tactic achieves the regulation for the Fermi energy and carrier concentration of graphene, and maintains the lattice structure of graphene at the same time. This research offers a general approach to the rational design of dipole molecules adsorbed on graphene for modulating its band gap. 2. Materials and methods 2.1. Preparation of the materials The three molecules (DR1P, DR1P’, and DRSP) with different dipole moments and orientations for the diverse functional groups of azobenzene can be attached on graphene via π-π interaction between the end pyrene and graphene. DR1P, DR1P’, and DRSP were synthesized as reported in previous literature [21]. All chemicals were purchased from Sigma-Aldrich and used as received. 4-R-aniline was dispersed in water yielding a suspension. A certain volume of aqueous HCl was added in the suspension to make a clear solution. A cold aqueous NaNO2 was added dropwise to the mixture and stirred in the ice bath. The resulting mixture was added to the solution of 2-(N-ethylanilino)ethanol in glacial acetic acid and water in an ice bath and then the aqueous Na2CO3 solution was added to adjust the pH to 6. The mid product was filtered and washed with hexane. The mid product, 1-pyrenebutryic acid, N,N′dicyclohexylcarbodiimide (DCC), and 4-(dimethylamino)pyridine (DMAP) (5:5:5:1) were mixed and stirred in anhydrous CHCl3. The crude solution was collected and concentrated with a rotary evaporator after the precipitate was filtered. The resulting liquid was further passed through a silica gel column with a CHCl3/hexane mixture as the eluent, collecting the resultant product (see Supporting Information Fig. S1). The single layer graphene for Raman spectroscopy was obtained by using scotch tape we started repeatedly peeling flakes of graphite off the mesas. When a Si wafer adheres to the scotch tape with the single layer graphene, some flakes became captured on the wafer’s surface. The Raman spectroscopy is performed on the Thermo Scientific DXRxi at 532 nm with the spot size of 5 μm and laser power of 10 mW. The peak position and intensity of Raman spectra is calibrated by SiO2 peak at 520 cm−1. So that it can be used in the comparison between different samples. Due to the small-scale graphene from mechanical exfoliation, the monolayer graphene prepared on copper foils (99.8%, 0.025 mm thick, Alfa Aesar) by chemical vapor deposition is used to fabricate field effect transistor for electrical measurements. Graphene growing on copper is deposited a protective poly(methyl methacrylate) (PMMA) and the original copper substrate is etched off by soaking in iron chloride (FeCl3, 0.2 M). The SiO2/Si substrate was subsequently cleaned by sonication in isopropanol, ethyl alcohol and acetone. Then the monolayer graphene is successfully transferred onto the insulating SiO2/Si substrate. The protective layer was removed by dissolving in acetone.
2.4. Physical and electrical characterization The layer number of graphene was determined by Raman spectrum. The azobenzene molecules (DR1P’, DR1P, and DRSP) were respectively dissolved in dichloromethane with concentrations ranging from 5 × 10−10 M to 5 × 10−5 M. A 20 μL of the azobenzene molecules solution from low concentration to high concentration were sequentially spin-coated on graphene at 3000 rpm for 30 s. The higher the concentration of azobenzene molecules solution spin-coated on the surface is, the more the azobenzene molecules on the surface of graphene kept. Raman spectra of azobenzene molecule/graphene samples were gained by Thermo Scientific DXR Raman microscope. Wavenumber and intensity were calibrated based on a Si peak at 520 cm−1. The electrical measurements were performed on the Keithley 4200 SCS and Signatone H100 series probe station. The graphene spin-coated with the azobenzene molecules is investigated its FET characteristics. 3. Results Fig. 1a–c show the structure of the three azobenzene molecules with distinct dipole moments and dipole orientations. They are all azobenzene derivatives attached various groups with the discrepant capacities of donating and withdrawing electron. The dipole moments of the DR1P’, DR1P and DRSP were calculated and defined to be −10.61 D, 13.55 D and −13.56 D, respectively. And the related calculated results are list in SI. The schematic diagram of the azobenzene molecules anchored to graphene via the π-π interaction between pyrene and graphene can be found in Fig. 1d. The Raman spectrum of the pristine single layer graphene in our study is measured and showed in Fig. 1e. The high ratio between 2D band and G band reveal that graphene is single layer and indistinct D band suggests the few defects. Meantime, the reproducibility of Raman spectrum for graphene was also conducted. As shown in Fig. S3, it can be observed that the peak positions as well as peak intensities are similar with each other at different regions on graphene. The standard deviation (σ) of peak shift on pristine graphene from different regions is 0.237 cm−1 (Table S1) which is far less than the shift from different modification (1.5–3 cm−1). Moreover, the experiments were operated in the same ambient atmosphere to the great extent. Hence, the small shift on the Raman peak for modified graphene can be clearly observed. Fig. 2a–c show the comparison between Raman intensity of the azobenzene molecules on graphene and that of azobenzene molecules on Si with the excitation wavelength of 532 nm. The three azobenzene molecules (DR1P’, DR1P and DRSP) were uniformly spin-coated at 3000 rpm for 30 s upon graphene attached on the Si wafer. The Raman intensity of the azobenzene molecules on graphene is several tens of times stronger than the corresponding value on the Si wafer regardless of its dipole moments and dipole orientations. As shown in Fig. 2d–f, the effect of the surface coverage on the Raman enhancement is investigated by changing the concentration (ranging from 5 × 10−10 to 5 × 10−5 M) of the three azobenzene molecules spin-coated on the
2.2. Computational details The following calculations were performed using the DFTB + package [22]. The geometric and electronic structure properties were studied with the dispersion-corrected self-consistent charge density functional tight-binding (SCC-DFTB-D) method [23,24]. The 3OB parameters set was chosen for the O, N, C, S and H elements contained in the organic molecules, and dispersion was included via a Lennard-Jones potential between each pair of atoms. 2.3. Fabrication procedure of graphene-based field effect transistors The field effect transistors based monolayer graphene were fabricated on the SiO2/Si substrate by UV photolithography process. The 901
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Fig. 1. The structure of the three azobenzene molecules with different dipole moments and dipole orientations (a) DR1P’, (b) DR1P, (c) DRSP. (d) Schematic illustration of the azobenzene molecules anchored to graphene via a pyrene tether. (e) The Raman spectrum of the pristine monolayer graphene.
solution increases, the G band further blue shift. When the concentration of the three azobenzene molecules (DR1P’, DR1P and DRSP) increase to 5 × 10−5 M, the G band of graphene separately blue shifts by 1.62, 2.74 and 2.69 cm−1, respectively. The blue shift from DR1P’/ graphene is lower than that from DR1P and DRSP spin-coating on graphene, which is in harmony with the dipole moment absolute values of the azobenzene molecules. And the blue shifts in G band of graphene become stronger as the concentration of azobenzene molecules increasing. The strong π-π interactions between the azobenzene molecules and graphene can cause an efficient n-/p- doping to graphene for a strong electronic coupling and charge transfer between the azobenzene molecules and graphene. The n-/p- doping to graphene stems from the electronic structure variation [31] of the azobenzene molecules/graphene hybrid. The reproducibility of G band Raman shift is investigated in Figs. S4–S6. It is observed that the variation tendency of G band with the increasing of azobenzene molecule concentration is similar with each other. Moreover, the 2D band Raman shift of graphene anchoring with the different concentration of the azobenzene molecules is also summarized. Unfortunately, the variation tendency of 2D band is messy which may be ascribed to that it is sensitivity to the ambient environment [14,32,33]. To give an insight into the electronic structure modulation of graphene modified by the azobenzene molecules, we further analyzed the G band shift of the graphene as the previous literature report [29]. The G band shifts are the direct correlation with the Fermi level. The G band blue shifted as the departure of Fermi level from Dirac point [34], owing to electron/hole (n-/p-) doping occurring (up/down shift in the Fermi level). In our results, the G band band shifts are both positively correlated with the concentration of the azobenzene molecules, which is shown in Fig. 4. The tendency of the fitting curve is analogous with the diagram of the Raman shift versus Fermi level [35], suggesting there are correlations between the concentration of the azobenzene molecules and Fermi level. Considering that the G band blue shifted greater as the concentration of the azobenzene molecules increasing, we could give a preliminary speculation that the modification of graphene from azobenzene molecules realizes n-/p- doping and alters the Fermi level of graphene. Since both n-type and p-type doping show the same directional changes in G band [36], it is tough to confirm the n- or p- doped system purely based on Raman spectra. To deeply understand the doping
graphene. The Raman intensity of the azobenzene molecules on graphene strengthens as the azobenzene molecular concentration increasing. The Raman enhancement may arise from the electronic coupling and hybridization between the azobenzene molecules and graphene via π-π interaction as mentioned prior. It has been reported that the charge transfer from the azobenzene molecules to graphene could induce efficient doping to graphene [25], thus the change of Raman intensity may be closely related with the Fermi level of graphene. Moreover, the normalized intensity of the molecular N]N stretching vibration mode (1389 cm−1) based on the Si peak (520 cm−1) was also observed in Fig. 2d–f. The increasing of Raman intensity becomes slower at the concentration of ∼1 × 10−5 M as shown in Fig. 2g–i, which indicates the saturation of the molecular surface coverage and corresponds with the Brunauer-Emmett-Teller model as the previous report mentioned [26–28]. The plots of the Raman intensity versus the concentration of the azobenzene molecules were shown in Fig. 2d–f. It can be described by the Langmuir nonlinear fitting as follows:
I = Imax × Cazobenzene/(Cazobenzene + M )
(1)
where I and Imax separately signify the specific Raman intensity of the azobenzene molecules and the maximum Raman intensity of the corresponding saturation adsorption, and Cazobenzene and M denote the concentration of the azobenzene molecule and the fitting parameter, respectively. The above analysis provides the possibility to apply the azobenzene molecules/graphene hybrids for quantitative surface enhancement Raman scattering (SERS) detection. The results demonstrate the electronic coupling and hybridization between the azobenzene molecules and graphene via π-π interaction. In order to further investigate the coupling and doping in the azobenzene molecules/graphene system, we analyzed the Raman shift variation of graphene in detail. It is well known that the G band Raman shift is susceptible to the carrier concentration [29] of graphene. Fig. 3 shows G band Raman spectrum of graphene spin-coated by azobenzene molecules (DR1P’, DR1P and DRSP) with different concentrations, respectively. The G band [30] of pristine graphene is correspondingly at around 1582 cm−1 and both blue shift a certain extent after spincoating 20 μL of azobenzene molecules solution with several different concentrations. As the concentration of the azobenzene molecules 902
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Fig. 2. Raman spectra comparison between the azobenzene molecules on graphene and azobenzene molecules on Si with the excitation wavelength of 532 nm, (a) DR1P’, (b) DR1P, (c) DRSP. (d-f) Raman spectra of different concentration (ranging from 5 × 10−10 to 5 × 10−5 M) of the azobenzene molecules (DR1P’, DR1P and DRSP) on graphene, asterisk (*) stand for the N = N stretching vibration mode at 1389 cm−1. (g-i) corresponding plots for the Raman intensity (calibrated by SiO2 peak at 520 cm−1) as a function of their concentration.
probe measurements were performed for the azobenzene molecules/ graphene devices with 3 V source-drain bias as a function of the gate voltage in the range of −10 to 10 V (Fig. S7). Herein, the discrepancy of Dirac point in the pristine graphene is ascribed to the volume of adsorption gas molecules and the contamination from transfer process
effects, we performed electrical measurements on graphene FET. The silicon wafer with SiO2 (300 nm) insulting layer were utilized as the gate electrode and dielectric for the graphene FET respectively, as shown in Fig. 5. The insert photograph of Fig. 5a displays the physical map of graphene FET and its illustration is showed in Fig. 5d. Three
Fig. 3. The changes in the G band Raman shift of graphene anchoring with the different concentration of the azobenzene molecules, (a) DR1P, (b) DR1P’ and (c) DRSP. 903
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Fig. 4. G band shifts of graphene with the concentration of the azobenzene molecules.
Fig. 5. (a–c) FET device characterization showing drain current as a function of gate voltage for pristine graphene and azobenzene molecules (DR1P’, DR1P, and DRSP)/graphene as prepared. (d) The schematic of the FET device based on the graphene through lithography.
the DR1P’ and DRSP. The doping effect imparity results from the dipole orientation diversity of the azobenzene molecules modifying on graphene. We normalize the G band shift and the Dirac point shift of the graphene before modified by the three azobenzene molecules via setting these parameters of pristine graphene as fixed value and zero. And on the basis of the relationship the G band position and carrier concentration of graphene in the previous literature [35], the carrier concentration estimation results of graphene are listed in Table 1. The carrier concentration alteration of graphene is consistent with the absolute value of dipole moment, proving that there exists charge transfer between the graphene and azobenzene molecules. The Dirac position variation is associated with the dipole moment and orientation of the three azobenzene molecules, demonstrating that the n- or p-type doping from the azobenzene molecules to graphene. However, the G band shows the same response to n- and p- doping and relate to the absolute value of Dirac shift. This phenomenon can be ascribed to that the larger Dirac shift will soften the electron–phonon coupling in graphene and
Table 1 G band shift and Dirac point shift of the graphene and carrier concentration (n) estimation from G band shift.
Dipole moment (D) G Band (cm−1) Δn (1013 cm−2) Dirac shift (V)
Graphene
DR1P′/G
DR1P/G
DRSP/G
– 1582.00 – 0.0
−10.61 1583.62 0.15 −0.6
13.55 1584.65 0.20 1.6
−13.56 1584.71 0.20 −1.7
[37–40]. As shown in Fig. 5a–c, the I-V characteristic curves of pristine graphene FET devices are normalized with a Dirac point of 0 V (the black line in Fig. 5a, b, and c). DR1P/graphene with Dirac point at 1.6 V shows obvious p-type behavior compared to pristine graphene. While the Dirac points of DR1P’/graphene and DRSP/graphene downshifts by 1.0 and 1.7 V, respectively, which indicates the depletion of the hole carriers and demonstrates the effective n-type doping to graphene from
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Fig. 6. A schematic of the graphene modified by the three azobenzene molecules and the Fermi level shifts of the graphene.
interaction. The results demonstrate the Raman shifts of graphene from the modification and Fermi level regulation of graphene via altering its carrier concentration. The characterization results of graphene FET validate the variation of Dirac point before and after modification, further confirming the n-/p- type doping of graphene from the three azobenzene molecules (DRSP, DR1P’ and DR1P) respectively. We believe that the modulating and regulating tactic in the Fermi level and carrier concentration of graphene via azobenzene molecules with different dipole moment and dipole orientation might promote the graphene application in nanoelectronics science in place of silicon-based material.
thus strengthen the G band photon energy [14]. The I-V curves for graphene and azobenzene molecules/graphene hybrid are also performed for three times as shown in Fig. S8 which demonstrates a good reproducibility. 4. Discussion Based on the discussion above, we have illustrated the modification of graphene from the azobenzene and the Fermi level regulation of graphene in Fig. 6. The electric structure of the pristine graphene is conical, but the actual electric structure of pure graphene is parabolic cone and there are some holes doping for the adsorption gas molecules and the contamination from transfer process in our research [37–40]. When the dipole orientation of azobenzene molecule modifying on the surface of graphene is reverse forward to graphene (DR1P), it is the effect of electron transfer from graphene to azobenzene molecule and cause distinct hole doping. In contrast, the azobenzene molecules with the dipole orientation forward to graphene (DR1P’ and DRSP) are modified on graphene, leading to electron doping of graphene. The bigger the absolute value of the dipole moment is, the more obviously the Fermi level moves up. The Raman shift and Dirac point alteration of graphene modified by the azobenzene molecules with different dipole moment and orientation can be ascribed to the n- or p-type doping and the move of the Fermi level of graphene. These results can help us further design novel graphene-based device by manipulating its energy band.
Acknowledgement This study was supported by the Natural Science Foundation of Shandong Province (China) for Distinguished Young Scholars (JQ201610), the Shandong Provincial Natural Science Foundation, China (ZR2016EMB18) and the Hundred Talents Program and Frontier Science Research Project (QYZDB-SSW-JSC052) of the Chinese Academy of Sciences.
Declarations of interest None.
5. Conclusion Appendix A. Supplementary material In summary, we have presented in detail the Raman spectrum of graphene modified by three azobenzene molecules (DR1P’, DR1P, and DRSP) with different dipole moment and dipole orientation via π-π
Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.apsusc.2018.09.021. 905
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Full Length Article
Tunable Fermi level of graphene modified by azobenzene molecules a,b,1
Jiaojiao Yu , Mingjia Zhang ⁎ Changshui Huanga,
a,1
a
c
d
T a
, Jianjiang He , Chunfang Zhang , Weiwei Cui , Ning Wang ,
a
Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, No. 189 Songling Road, Qingdao 266101, PR China University of Chinese Academy of Sciences, Beijing 100190, PR China Department of Chemistry and Environmental Science, Hebei University, Baoding 071002, PR China d Department of Physics, Qingdao University, Qingdao 266071, PR China b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Graphene Raman spectroscopy Field-effect transistor Fermi level Carrier concentration
Carbon-based nanomaterials, especially graphene, are considered to be the most hopeful alternatives to silicon in the near future of pushing to its limits. However, the zero band gap of graphene suppresses its application, it needs to develop a convenient method to modulate its Fermi level and band gap. Herein, we report a general approach to regulate Fermi level of graphene precisely by designed azobenzene molecules with different dipole moment and dipole orientation. The Raman spectra results demonstrate the π-π interaction between the azobenzene molecules and graphene, which result in the modulation of the Fermi level of graphene. Besides, based on the field effect transistor characteristic measurement, we also observe the Fermi level adjustment for the hole/electron doping to graphene from the spontaneous polarization effect of the azobenzene molecules, further speculating the regulation of electronic structure and providing a new route for changing the electrical properties of graphene.
1. Introduction
Graphene has four half-filled degenerate states at the intrinsic Fermi level which consisting of two degenerate states at two nonequivalent Dirac points (K and K’), comes from the crystal symmetry of graphene's honeycomb lattice. Band gap opening in graphene thus implies breaking of the symmetry [13]. Hence, many efforts have been dedicated to open the band gap of graphene by modulating the Dirac point or Fermi level [14]. It is reported that the graphene Fermi level is modulated by electrical field effect which would result in hole or electron doping [15]. Graphene nanoribbons and atom-doped graphene are also investigated for the application in field effect transistors which demonstrate high on-off ratios [16]. Just recently, the molecules (including gas, molecular acceptor, self-assembled monolayers) adsorbed on graphene are considered to be an effective way to tune the electronic structure of intrinsic graphene because of its harmlessness, simpleness and effectivity [17–20]. However, general rules for the adjustment of graphene Fermi level by adsorbed molecules have not yet been established. Herein, we modify the surface of graphene with the azobenzene molecules with different dipole moments and dipole orientations to realize the n- and p- doping and regulate its Fermi level. Raman spectrum and field effect transistor (FET) are used to characterize the
The foundation of modern information technology is the integrated circuit chip, 90% of which is silicon-based complementary metal-oxidesemiconductor (CMOS) technology. Silicon-based CMOS technology is about to enter the 14 nm node after half a century rapid development and will reach its performance limit [1]. With the increasingly critical research in the post-Moore era of nanoelectronics science, many researchers count on the development of non-silicon-based electronics and abandon the use of silicon as a supporting material to be up against the 8 nm technology node. Among the few possible candidate materials [1–3], carbon-based nanomaterials, especially graphene [4,5], are considered to be the most hopeful alternatives [6] to silicon due to its extremely small thickness and extraordinary carrier mobility [7] and will likely profoundly affect the future development of chips and related industries. Since the discovery of graphene in 2004 [8], it becomes a fascinating material [5] applying to the field of high-frequency transistors, sensors, flexible electronics for its great mechanical strength [9], excellent chemical and thermal stability [10], and brilliant flexibility [11,12]. Nevertheless, the zero band gap severely limits its application in the semiconductor field.
⁎
Corresponding author. E-mail address: [email protected] (C. Huang). 1 These authors contributed equally. https://doi.org/10.1016/j.apsusc.2018.09.021 Received 10 February 2018; Received in revised form 30 August 2018; Accepted 3 September 2018 Available online 04 September 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved.
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surface was cleaned by UV/ozone treatment to remove the contamination from the surface of the wafer and graphene. The photoresist (AR-P 3740) was spin-coated on the surface of graphene at high-speed (1500 rpm) for 30 s to result in a uniform thin layer between 2 and 3 µm thick. Preparation of the resist is concluded prebaking over a hot plate at 100 °C for 30 s to evaporate the photoresist solvent. After prebaking, the sample coated photoresist was exposed to a pattern of intense ultraviolet light within the intensity of 100 mJ/cm2 through photolithographic masks. The samples were developed in developer for 30 s and rinsed using deionized water. The sample was then post-baked to drive off excess water at 120 °C for 60 s on a hotplate. Silver (80 nm) electrode was deposited on the graphene by evaporation. And lift-off process was performed in toluene with immersion times typically of several hours (see Supporting Information Fig. S2).
interaction between the azobenzene molecules and graphene and the modulation in the Fermi level of graphene. The results demonstrate that the tactic achieves the regulation for the Fermi energy and carrier concentration of graphene, and maintains the lattice structure of graphene at the same time. This research offers a general approach to the rational design of dipole molecules adsorbed on graphene for modulating its band gap. 2. Materials and methods 2.1. Preparation of the materials The three molecules (DR1P, DR1P’, and DRSP) with different dipole moments and orientations for the diverse functional groups of azobenzene can be attached on graphene via π-π interaction between the end pyrene and graphene. DR1P, DR1P’, and DRSP were synthesized as reported in previous literature [21]. All chemicals were purchased from Sigma-Aldrich and used as received. 4-R-aniline was dispersed in water yielding a suspension. A certain volume of aqueous HCl was added in the suspension to make a clear solution. A cold aqueous NaNO2 was added dropwise to the mixture and stirred in the ice bath. The resulting mixture was added to the solution of 2-(N-ethylanilino)ethanol in glacial acetic acid and water in an ice bath and then the aqueous Na2CO3 solution was added to adjust the pH to 6. The mid product was filtered and washed with hexane. The mid product, 1-pyrenebutryic acid, N,N′dicyclohexylcarbodiimide (DCC), and 4-(dimethylamino)pyridine (DMAP) (5:5:5:1) were mixed and stirred in anhydrous CHCl3. The crude solution was collected and concentrated with a rotary evaporator after the precipitate was filtered. The resulting liquid was further passed through a silica gel column with a CHCl3/hexane mixture as the eluent, collecting the resultant product (see Supporting Information Fig. S1). The single layer graphene for Raman spectroscopy was obtained by using scotch tape we started repeatedly peeling flakes of graphite off the mesas. When a Si wafer adheres to the scotch tape with the single layer graphene, some flakes became captured on the wafer’s surface. The Raman spectroscopy is performed on the Thermo Scientific DXRxi at 532 nm with the spot size of 5 μm and laser power of 10 mW. The peak position and intensity of Raman spectra is calibrated by SiO2 peak at 520 cm−1. So that it can be used in the comparison between different samples. Due to the small-scale graphene from mechanical exfoliation, the monolayer graphene prepared on copper foils (99.8%, 0.025 mm thick, Alfa Aesar) by chemical vapor deposition is used to fabricate field effect transistor for electrical measurements. Graphene growing on copper is deposited a protective poly(methyl methacrylate) (PMMA) and the original copper substrate is etched off by soaking in iron chloride (FeCl3, 0.2 M). The SiO2/Si substrate was subsequently cleaned by sonication in isopropanol, ethyl alcohol and acetone. Then the monolayer graphene is successfully transferred onto the insulating SiO2/Si substrate. The protective layer was removed by dissolving in acetone.
2.4. Physical and electrical characterization The layer number of graphene was determined by Raman spectrum. The azobenzene molecules (DR1P’, DR1P, and DRSP) were respectively dissolved in dichloromethane with concentrations ranging from 5 × 10−10 M to 5 × 10−5 M. A 20 μL of the azobenzene molecules solution from low concentration to high concentration were sequentially spin-coated on graphene at 3000 rpm for 30 s. The higher the concentration of azobenzene molecules solution spin-coated on the surface is, the more the azobenzene molecules on the surface of graphene kept. Raman spectra of azobenzene molecule/graphene samples were gained by Thermo Scientific DXR Raman microscope. Wavenumber and intensity were calibrated based on a Si peak at 520 cm−1. The electrical measurements were performed on the Keithley 4200 SCS and Signatone H100 series probe station. The graphene spin-coated with the azobenzene molecules is investigated its FET characteristics. 3. Results Fig. 1a–c show the structure of the three azobenzene molecules with distinct dipole moments and dipole orientations. They are all azobenzene derivatives attached various groups with the discrepant capacities of donating and withdrawing electron. The dipole moments of the DR1P’, DR1P and DRSP were calculated and defined to be −10.61 D, 13.55 D and −13.56 D, respectively. And the related calculated results are list in SI. The schematic diagram of the azobenzene molecules anchored to graphene via the π-π interaction between pyrene and graphene can be found in Fig. 1d. The Raman spectrum of the pristine single layer graphene in our study is measured and showed in Fig. 1e. The high ratio between 2D band and G band reveal that graphene is single layer and indistinct D band suggests the few defects. Meantime, the reproducibility of Raman spectrum for graphene was also conducted. As shown in Fig. S3, it can be observed that the peak positions as well as peak intensities are similar with each other at different regions on graphene. The standard deviation (σ) of peak shift on pristine graphene from different regions is 0.237 cm−1 (Table S1) which is far less than the shift from different modification (1.5–3 cm−1). Moreover, the experiments were operated in the same ambient atmosphere to the great extent. Hence, the small shift on the Raman peak for modified graphene can be clearly observed. Fig. 2a–c show the comparison between Raman intensity of the azobenzene molecules on graphene and that of azobenzene molecules on Si with the excitation wavelength of 532 nm. The three azobenzene molecules (DR1P’, DR1P and DRSP) were uniformly spin-coated at 3000 rpm for 30 s upon graphene attached on the Si wafer. The Raman intensity of the azobenzene molecules on graphene is several tens of times stronger than the corresponding value on the Si wafer regardless of its dipole moments and dipole orientations. As shown in Fig. 2d–f, the effect of the surface coverage on the Raman enhancement is investigated by changing the concentration (ranging from 5 × 10−10 to 5 × 10−5 M) of the three azobenzene molecules spin-coated on the
2.2. Computational details The following calculations were performed using the DFTB + package [22]. The geometric and electronic structure properties were studied with the dispersion-corrected self-consistent charge density functional tight-binding (SCC-DFTB-D) method [23,24]. The 3OB parameters set was chosen for the O, N, C, S and H elements contained in the organic molecules, and dispersion was included via a Lennard-Jones potential between each pair of atoms. 2.3. Fabrication procedure of graphene-based field effect transistors The field effect transistors based monolayer graphene were fabricated on the SiO2/Si substrate by UV photolithography process. The 901
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Fig. 1. The structure of the three azobenzene molecules with different dipole moments and dipole orientations (a) DR1P’, (b) DR1P, (c) DRSP. (d) Schematic illustration of the azobenzene molecules anchored to graphene via a pyrene tether. (e) The Raman spectrum of the pristine monolayer graphene.
solution increases, the G band further blue shift. When the concentration of the three azobenzene molecules (DR1P’, DR1P and DRSP) increase to 5 × 10−5 M, the G band of graphene separately blue shifts by 1.62, 2.74 and 2.69 cm−1, respectively. The blue shift from DR1P’/ graphene is lower than that from DR1P and DRSP spin-coating on graphene, which is in harmony with the dipole moment absolute values of the azobenzene molecules. And the blue shifts in G band of graphene become stronger as the concentration of azobenzene molecules increasing. The strong π-π interactions between the azobenzene molecules and graphene can cause an efficient n-/p- doping to graphene for a strong electronic coupling and charge transfer between the azobenzene molecules and graphene. The n-/p- doping to graphene stems from the electronic structure variation [31] of the azobenzene molecules/graphene hybrid. The reproducibility of G band Raman shift is investigated in Figs. S4–S6. It is observed that the variation tendency of G band with the increasing of azobenzene molecule concentration is similar with each other. Moreover, the 2D band Raman shift of graphene anchoring with the different concentration of the azobenzene molecules is also summarized. Unfortunately, the variation tendency of 2D band is messy which may be ascribed to that it is sensitivity to the ambient environment [14,32,33]. To give an insight into the electronic structure modulation of graphene modified by the azobenzene molecules, we further analyzed the G band shift of the graphene as the previous literature report [29]. The G band shifts are the direct correlation with the Fermi level. The G band blue shifted as the departure of Fermi level from Dirac point [34], owing to electron/hole (n-/p-) doping occurring (up/down shift in the Fermi level). In our results, the G band band shifts are both positively correlated with the concentration of the azobenzene molecules, which is shown in Fig. 4. The tendency of the fitting curve is analogous with the diagram of the Raman shift versus Fermi level [35], suggesting there are correlations between the concentration of the azobenzene molecules and Fermi level. Considering that the G band blue shifted greater as the concentration of the azobenzene molecules increasing, we could give a preliminary speculation that the modification of graphene from azobenzene molecules realizes n-/p- doping and alters the Fermi level of graphene. Since both n-type and p-type doping show the same directional changes in G band [36], it is tough to confirm the n- or p- doped system purely based on Raman spectra. To deeply understand the doping
graphene. The Raman intensity of the azobenzene molecules on graphene strengthens as the azobenzene molecular concentration increasing. The Raman enhancement may arise from the electronic coupling and hybridization between the azobenzene molecules and graphene via π-π interaction as mentioned prior. It has been reported that the charge transfer from the azobenzene molecules to graphene could induce efficient doping to graphene [25], thus the change of Raman intensity may be closely related with the Fermi level of graphene. Moreover, the normalized intensity of the molecular N]N stretching vibration mode (1389 cm−1) based on the Si peak (520 cm−1) was also observed in Fig. 2d–f. The increasing of Raman intensity becomes slower at the concentration of ∼1 × 10−5 M as shown in Fig. 2g–i, which indicates the saturation of the molecular surface coverage and corresponds with the Brunauer-Emmett-Teller model as the previous report mentioned [26–28]. The plots of the Raman intensity versus the concentration of the azobenzene molecules were shown in Fig. 2d–f. It can be described by the Langmuir nonlinear fitting as follows:
I = Imax × Cazobenzene/(Cazobenzene + M )
(1)
where I and Imax separately signify the specific Raman intensity of the azobenzene molecules and the maximum Raman intensity of the corresponding saturation adsorption, and Cazobenzene and M denote the concentration of the azobenzene molecule and the fitting parameter, respectively. The above analysis provides the possibility to apply the azobenzene molecules/graphene hybrids for quantitative surface enhancement Raman scattering (SERS) detection. The results demonstrate the electronic coupling and hybridization between the azobenzene molecules and graphene via π-π interaction. In order to further investigate the coupling and doping in the azobenzene molecules/graphene system, we analyzed the Raman shift variation of graphene in detail. It is well known that the G band Raman shift is susceptible to the carrier concentration [29] of graphene. Fig. 3 shows G band Raman spectrum of graphene spin-coated by azobenzene molecules (DR1P’, DR1P and DRSP) with different concentrations, respectively. The G band [30] of pristine graphene is correspondingly at around 1582 cm−1 and both blue shift a certain extent after spincoating 20 μL of azobenzene molecules solution with several different concentrations. As the concentration of the azobenzene molecules 902
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Fig. 2. Raman spectra comparison between the azobenzene molecules on graphene and azobenzene molecules on Si with the excitation wavelength of 532 nm, (a) DR1P’, (b) DR1P, (c) DRSP. (d-f) Raman spectra of different concentration (ranging from 5 × 10−10 to 5 × 10−5 M) of the azobenzene molecules (DR1P’, DR1P and DRSP) on graphene, asterisk (*) stand for the N = N stretching vibration mode at 1389 cm−1. (g-i) corresponding plots for the Raman intensity (calibrated by SiO2 peak at 520 cm−1) as a function of their concentration.
probe measurements were performed for the azobenzene molecules/ graphene devices with 3 V source-drain bias as a function of the gate voltage in the range of −10 to 10 V (Fig. S7). Herein, the discrepancy of Dirac point in the pristine graphene is ascribed to the volume of adsorption gas molecules and the contamination from transfer process
effects, we performed electrical measurements on graphene FET. The silicon wafer with SiO2 (300 nm) insulting layer were utilized as the gate electrode and dielectric for the graphene FET respectively, as shown in Fig. 5. The insert photograph of Fig. 5a displays the physical map of graphene FET and its illustration is showed in Fig. 5d. Three
Fig. 3. The changes in the G band Raman shift of graphene anchoring with the different concentration of the azobenzene molecules, (a) DR1P, (b) DR1P’ and (c) DRSP. 903
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Fig. 4. G band shifts of graphene with the concentration of the azobenzene molecules.
Fig. 5. (a–c) FET device characterization showing drain current as a function of gate voltage for pristine graphene and azobenzene molecules (DR1P’, DR1P, and DRSP)/graphene as prepared. (d) The schematic of the FET device based on the graphene through lithography.
the DR1P’ and DRSP. The doping effect imparity results from the dipole orientation diversity of the azobenzene molecules modifying on graphene. We normalize the G band shift and the Dirac point shift of the graphene before modified by the three azobenzene molecules via setting these parameters of pristine graphene as fixed value and zero. And on the basis of the relationship the G band position and carrier concentration of graphene in the previous literature [35], the carrier concentration estimation results of graphene are listed in Table 1. The carrier concentration alteration of graphene is consistent with the absolute value of dipole moment, proving that there exists charge transfer between the graphene and azobenzene molecules. The Dirac position variation is associated with the dipole moment and orientation of the three azobenzene molecules, demonstrating that the n- or p-type doping from the azobenzene molecules to graphene. However, the G band shows the same response to n- and p- doping and relate to the absolute value of Dirac shift. This phenomenon can be ascribed to that the larger Dirac shift will soften the electron–phonon coupling in graphene and
Table 1 G band shift and Dirac point shift of the graphene and carrier concentration (n) estimation from G band shift.
Dipole moment (D) G Band (cm−1) Δn (1013 cm−2) Dirac shift (V)
Graphene
DR1P′/G
DR1P/G
DRSP/G
– 1582.00 – 0.0
−10.61 1583.62 0.15 −0.6
13.55 1584.65 0.20 1.6
−13.56 1584.71 0.20 −1.7
[37–40]. As shown in Fig. 5a–c, the I-V characteristic curves of pristine graphene FET devices are normalized with a Dirac point of 0 V (the black line in Fig. 5a, b, and c). DR1P/graphene with Dirac point at 1.6 V shows obvious p-type behavior compared to pristine graphene. While the Dirac points of DR1P’/graphene and DRSP/graphene downshifts by 1.0 and 1.7 V, respectively, which indicates the depletion of the hole carriers and demonstrates the effective n-type doping to graphene from
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Fig. 6. A schematic of the graphene modified by the three azobenzene molecules and the Fermi level shifts of the graphene.
interaction. The results demonstrate the Raman shifts of graphene from the modification and Fermi level regulation of graphene via altering its carrier concentration. The characterization results of graphene FET validate the variation of Dirac point before and after modification, further confirming the n-/p- type doping of graphene from the three azobenzene molecules (DRSP, DR1P’ and DR1P) respectively. We believe that the modulating and regulating tactic in the Fermi level and carrier concentration of graphene via azobenzene molecules with different dipole moment and dipole orientation might promote the graphene application in nanoelectronics science in place of silicon-based material.
thus strengthen the G band photon energy [14]. The I-V curves for graphene and azobenzene molecules/graphene hybrid are also performed for three times as shown in Fig. S8 which demonstrates a good reproducibility. 4. Discussion Based on the discussion above, we have illustrated the modification of graphene from the azobenzene and the Fermi level regulation of graphene in Fig. 6. The electric structure of the pristine graphene is conical, but the actual electric structure of pure graphene is parabolic cone and there are some holes doping for the adsorption gas molecules and the contamination from transfer process in our research [37–40]. When the dipole orientation of azobenzene molecule modifying on the surface of graphene is reverse forward to graphene (DR1P), it is the effect of electron transfer from graphene to azobenzene molecule and cause distinct hole doping. In contrast, the azobenzene molecules with the dipole orientation forward to graphene (DR1P’ and DRSP) are modified on graphene, leading to electron doping of graphene. The bigger the absolute value of the dipole moment is, the more obviously the Fermi level moves up. The Raman shift and Dirac point alteration of graphene modified by the azobenzene molecules with different dipole moment and orientation can be ascribed to the n- or p-type doping and the move of the Fermi level of graphene. These results can help us further design novel graphene-based device by manipulating its energy band.
Acknowledgement This study was supported by the Natural Science Foundation of Shandong Province (China) for Distinguished Young Scholars (JQ201610), the Shandong Provincial Natural Science Foundation, China (ZR2016EMB18) and the Hundred Talents Program and Frontier Science Research Project (QYZDB-SSW-JSC052) of the Chinese Academy of Sciences.
Declarations of interest None.
5. Conclusion Appendix A. Supplementary material In summary, we have presented in detail the Raman spectrum of graphene modified by three azobenzene molecules (DR1P’, DR1P, and DRSP) with different dipole moment and dipole orientation via π-π
Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.apsusc.2018.09.021. 905
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