Evidence for formation of multi-quantum dots in hydrogenated graphene

  • Chiashain Chuang1, 2,

    Affiliated with

    • Reuben K Puddy1,

      Affiliated with

      • Malcolm R Connolly1,

        Affiliated with

        • Shun-Tsung Lo3,

          Affiliated with

          • Huang-De Lin2,

            Affiliated with

            • Tse-Ming Chen4,

              Affiliated with

              • Charles G Smith1Email author and

                Affiliated with

                • Chi-Te Liang2, 3Email author

                  Affiliated with

                  Nanoscale Research Letters20127:459

                  DOI: 10.1186/1556-276X-7-459

                  Received: 26 June 2012

                  Accepted: 10 August 2012

                  Published: 16 August 2012


                  We report the experimental evidence for the formation of multi-quantum dots in a hydrogenated single-layer graphene flake. The existence of multi-quantum dots is supported by the low-temperature measurements on a field effect transistor structure device. The resulting Coulomb blockade diamonds shown in the color scale plot together with the number of Coulomb peaks exhibit the characteristics of the so-called ‘stochastic Coulomb blockade’. A possible explanation for the formation of the multi-quantum dots, which is not observed in pristine graphene to date, was attributed to the impurities and defects unintentionally decorated on a single-layer graphene flake which was not treated with the thermal annealing process. Graphene multi-quantum dots developed around impurities and defect sites during the hydrogen plasma exposure process.


                  Multi-quantum dots Single-layer graphene flake Coulomb peaks


                  Graphene, a mono-layer of carbon atoms arranged in a honeycomb lattice, has extraordinary electrical properties, such as the gapless linear dispersion[14]. In order to realize graphene-based nanoelectronic device applications, many research groups tried to open the energy bandgap in the gapless linear dispersion in different ways, for instance, graphene nanoribbons[5, 6] and bilayer graphene applied by the electric field[79]. Recently, hydrogenated graphene attracts a great deal of attention because of its bandgap behavior driven by the chemical functionalization[1017]. The adsorbed atomic hydrogen atoms form three-dimensional C-H sp3 covalent bonds with carbon atoms by interrupting C-C sp2 bonds, thus, removing the conducting π bonds and opening a bandgap[11, 18, 19]. In 2010, Singh and co-workers proposed that graphane could form natural host for graphene multi-quantum dots, clusters of vacancies in hydrogen sublattice[20]. According to the surface dynamics, thermally energetic hydrogen atoms adsorbed on graphene could be desorbed from the graphene surface or migrate to the proper bonding sites or nucleate randomly (due to short diffusion length) to form dense islands of coexisting two-dimensional phases, C-H and C-C[14, 20, 21]. On the other hand, some reports proposed that the multi-quantum dots were unintentionally formed by impurities or defects in single-wall carbon nanotubes, which belong to the same honeycomb lattice as single-layer graphene[2224].

                  In this study, we propose a possible explanation based on the aforementioned mechanism for the formation of multi-quantum dots on our single-layer graphene flake and supported by the low-temperature electrical transport measurements.


                  A graphene field-effect transistor (FET) device was fabricated for the investigation described in this work. A single-layer graphene flake, mechanically exfoliated from natural graphite, was deposited onto a highly doped Si substrate capped with a 300-nm-thick SiO2 layer, serving as a back gate[25]. Optical microscopy was used to locate graphene flakes and confirms that it was a single layer shown in the inset to Figure 1a[1, 25]. Two Ti/Au contacts (5/50 nm) were patterned, using e-beam lithography and lift-off processing, into the source and drain contacts. To retain the defects and impurities in the graphene flakes to facilitate the formation of multi-quantum dots, the FET device was conditioned by the hydrogen plasma at conditions of power = 16 W and pressure = 0.2 Torr for 6 s without post-exfoliation annealing treatment[10, 26].
                  Figure 1

                  Source-drain current ( I SD ) dependence. (a) ISD measured at VBG from VBG = 0 to 85 V at 1.32 K with a fixed source and drain voltage, VSD = 0.1 mV, before hydrogen plasma treatment. The neutrality point voltage VNP is near 74 V. Inset: the optical image of a single-layer graphene flake in contrast (b) ISD measured from VBG = −50 to 110 V at T = 1.41 K with a fixed source and drain voltage VSD = 20 mV after hydrogen plasma treatment. The Coulomb blockade oscillations occur between 30 and 50 V. Inset: the Coulomb peaks at T = 1.32 K with a fixed source and drain voltage VSD = 1 mV.

                  An Oxford top-loading He4 cryostat was used to carry out the two-terminal conductance measurements using standard AC lock-in technique at 77 Hz with a DC bias at the temperature range between 1.3 and 40 K.

                  Results and discussion

                  Figure 1a shows the source-drain current (ISD) dependence on the back gate voltage (VBG) measured at the charge neutrality point, VNP = 74 V, with a fixed source-drain voltage VSD = 0.1 mV at T = 1.32 K before the hydrogen plasma treatment. The charge neutrality point, which is far from the zero voltage, can be attributed to the hole-doping impurities left on the graphene flake[27, 28]. Figure 1b shows the ISDVBG measurement after hydrogen plasma treatment. Strong suppression of the source-drain current in the Coulomb blockade oscillation region (between the dashed lines) with a fixed source-drain voltage VSD = 20 mV at T = 1.41 K is observed. To assure the Coulomb peaks in the Coulomb blockade oscillation region, we examined the Coulomb peaks with a fixed VSD = 1 mV at T = 1.32 K shown in the inset to Figure 1b[29]. To further investigate the Coulomb blockade effect, the Coulomb blockade color scale plot of the conductance G in a VBGVSD plane was adopted for a better illustration of the existence of multi-quantum dots in our graphene flake sample; overlapped diamond-shape pattern was expected.

                  Figure 2 shows a color scale plot of the differential conductance G versus VBG and VSD at T = 5 K. The overlap of Coulomb diamonds, so-called ‘Coulomb shards’, was observed[30]. The Coulomb shards, which is also called stochastic Coulomb blockade, occurred due to the multi-quantum dots coupling in series during the carrier transport tunneling process[3033]. Results of the measurements indicated that the multi-quantum dots formed in a two-dimensional manner. In other words, carriers could tunnel through the potential barriers of the quantum dots dispersed randomly. Coulomb shards disappeared while the temperature was increased to T = 10 K as shown in Figure 2b, whereby it implied that thermal energy dominated the carrier transport behavior rather than the multi-quantum dot Coulomb blockade tunneling[3133].
                  Figure 2

                  Color scale plot of the conductance G versus V BG and V SD . Shown at (a) T = 5 K and (b) T = 10 K. The back gate voltage swept from 40 to 45 V at a step of 100 mV. The irregular feature of the Coulomb blockade region in Figure 2a suggests a multi-quantum dot formation.

                  The stochastic Coulomb blockade in the multi-quantum dot system is further supported by investigating the temperature dependence of the number of the Coulomb peaks. Figure 3a shows the differential conductance as a function of VBG between 40.5 and 44.5 V at different temperatures with a fixed VSD = 9.5 mV. To distinguish the real Coulomb blockade peaks from the background noise, only reproducible peaks observed at the same VBG with varying VSD (VSD = 6.5, 7.5, and 9.5 mV) are considered, shown in the inset to Figure 3a. The oscillations in Figure 3a are non-periodic, and the number of Coulomb peaks increases monotonically as the temperature is increased as shown in Figure 3b[22, 30, 31]. Both the aforementioned are the typical characteristics of the stochastic Coulomb blockade which suggests a formation of multi-quantum dots[3134].
                  Figure 3

                  Temperature dependence and the number of Coulomb peaks. (a) Temperature dependence of G versus VBG (Coulomb oscillations) at VSD = 9.5 mV. Coulomb peaks are defined by the ones that were consistently reproduced at different VSD whereas at the same VBG as illustrated in the inset to Figure 3a. (b) The number of Coulomb peaks as a function of the temperature corresponds to those depicted in Figure 3a.

                  For a better visualization of the individual Coulomb diamond in the blockade region, the Coulomb diamond color scale plot of the conductance G with a better resolution ΔVBG = 10 mV at T = 6.5 K was shown in Figure 4. The clear Coulomb diamonds indicated that the charging effect existed in our hydrogenated graphene system[3537].
                  Figure 4

                  Color scale plot of the conductance G versus V BG and V SD at T= 6.5 K. The VBG was increased from 40 to 45 V at a step of 10 mV.

                  To justify the revealed overlapped Coulomb diamonds in our hydrogenated graphene system, a possible explanation for the formation of the multi-quantum dots is depicted in Figure 5. Without the post-exfoliation annealing process, the impurities or/and as-grown defects, shown as dots in Figure 5a, existed on the single-layer graphene flake[3840]. In the vicinity of defects (mostly vacancies) or impurities, hydrogen passivated the edge carbon atoms on the vacancy sites or substituted impurities by keeping the C-C sp2 bonding structure. In the defect/impurity-free regions, the C-H bonding transformed the C-C bonding from sp2 into sp3 structure[10, 26]. After hydrogen plasma exposure, graphene multi-quantum dots were formed in the proximity of defects/impurities, depicted in Figure 5b. The asymmetric hydrogenated graphene quantum dot array could be treated as the sequential tunneling of charges through the two-dimensional (2D) array of single-layer graphene quantum dots[41]. The experimental results indicated that 2D multi-quantum dot array can be achieved by the hydrogenation of exfoliated graphene flakes experiencing no annealing process. More detailed fundamental understanding of the origin of multi-quantum dots formed on the non-annealed hydrogenated graphene flakes can greatly promote the development of graphene-based multi-quantum dot devices for quantum computation[42, 43].
                  Figure 5

                  Schematics of defects and impurities and the formation of multi-quantum dots. (a) Schematic of defects and impurities on a single-layer graphene flake before hydrogen plasma treatment. (b) Schematic of the formation of multi-quantum dots on hydrogen graphene. The white regions, containing the defects and impurities, enclosed by the hydrogen atoms (the green dots) represent graphene multi-quantum dots.


                  Two-dimensional multi-quantum dots can be realized on a mechanically exfoliated graphene flake followed by the hydrogen plasma treatment without executing post-exfoliation thermal annealing. The overlapped Coulomb blockade diamonds observed from the electrical measurements, as well as the monotonic increase of the number of Coulomb peaks with the ascending temperature, suggest the formation of two-dimensional multi-quantum dots that is unprecedented on the annealed graphene flakes with similar hydrogenation processes. Therefore, we suggest a defect (or vacancy) and impurity-related mechanism to account for the formation of the multi-quantum dots discovered on our device. Further characterizations, such as AFM or SEM, on the atomic structure of un-annealed graphene layers might shed light on the origin of the quantum dot formation, whereas the degree of post-growth annealing could be utilized to engineer the quantum dots in terms of its size, density, shape, or charging states in a cost-effective way for quantum chip device applications.

                  Authors’ information

                  CC obtained his B.Sc. degree in Physics at NCUE in 2006 and M.Sc. degree in Physics at NTNU in 2009. He is currently pursuing his Ph.D. degree in Physics at NTU. RKP is currently pursuing his Ph.D. degree at the Cavendish Laboratory, University of Cambridge. MRC is currently a postdoctoral research worker at the Cavendish Laboratory, University of Cambridge. STL obtained his B.Sc. degree at NTU in 2010 and is pursuing his Ph.D. degree at the Graduate Institute of Applied Physics, NTU. He won the Dr. An-Tai Chen Scholarship, Mr. Ming Kao Scholarship, and college students participating in special research project of Creative Award provided by the NSC in 2009. HDL obtained his B.S. degree at Chinese Culture University, Taiwan and his Ph.D. degree at Mississippi State University, USA, and currently works as a project engineer at Electronics Testing Center, Tao-Yuan, Taiwan (R.O.C). TMC obtained his B.Sc. degree and M.Sc. degree at NTU, Taiwan and obtained his Ph.D. degree at Cambridge University, UK. He is currently an assistant professor at the Department of Physics, NCKU. CGS obtained his Ph.D. degree at Cambridge University, UK and is currently a professor of Physics at Cambridge University, UK. CTL obtained his B.Sc. degree at NTU in 1990 and his Ph.D. degree in Physics at Cambridge University, UK in 1996 and is currently a professor of Physics at NTU. He is also a topical editor for Current Applied Physics.



                  field-effect transistor





                  This work was funded by the Initiative Research Cooperation among top universities between UK and Taiwan (grant no.: NSC 99-2911-I-002-126), the NSC (grant no: NSC 101-2923-M-002-003-MY3), and National Taiwan University (grant no: 101R7552-2). CC, TMC, and CTL would like to thank the hospitality of the Semiconductor Physics Group, Cavendish Laboratory. CTL thanks Tina Liang, Valen Liang, and Eva Liang for their support.

                  Authors’ Affiliations

                  Cavendish Laboratory, University of Cambridge
                  Department of Physics, National Taiwan University
                  Graduate Institute of Applied Physics, National Taiwan University
                  Department of Physics, National Cheng Kung University


                  1. Geim AK, Novoselov KS: The rise of graphene. Nat Mate 2007, 6: 183.View Article
                  2. Castro Neto AH, Guinea F, Peres NMR, Novoselov KS, Geim AK: The electronic properties of graphene. Rev Mod Phys 2009, 81: 109. 10.1103/RevModPhys.81.109View Article
                  3. Zhao S, Lv Y, Yang X: Layer-dependent nanoscale electrical properties of graphene studied by conductive scanning probe microscopy. Nano Res Letts 2011, 6: 498. 10.1186/1556-276X-6-498View Article
                  4. Ishikawa R, Bando M, Morimoto Y, Sandhu A: Doping graphene films via chemically mediated charge transfer. Nano Res Letts 2011, 6: 111. 10.1186/1556-276X-6-111View Article
                  5. Han MY, Özyilmaz B, Zhang Y, Kim P: Energy band-gap engineering of graphene nanoribbons. Phys Rev Lett 2007, 98: 206805.View Article
                  6. Son Y-W, Cohen ML, Louie SG: Energy gaps in graphene nanoribbons. Phys Rev Lett 2006, 97: 216803.View Article
                  7. Castro EV, Novoselov KS, Morozov SV, Peres NMR, Santos JMB L, Nilsson J, Guinea F, Geim AK, Castro Neto AH: Biased bilayer graphene: semiconductor with a gap tunable by the electric field effect. Phys Rev Lett 2007, 99: 216802.View Article
                  8. Ohta T, Bostwick A, Seyller T, Horn K, Rotenberg E: Controlling the electronic structure of bilayer graphene. Science 2006, 313: 951. 10.1126/science.1130681View Article
                  9. Oostinga JB, Heersche HB, Liu X, Morpurgo AF, Vandersypen LMK: Gate-induced insulating state in bilayer graphene devices. Nature 2008, 7: 151. 10.1038/nmat2082View Article
                  10. Elias DC, Nair RR, Mohiuddin TMG, Morozov SV, Blake P, Halsall MP, Ferrari AC, Boukhvalov DW, Katsnelson MI, Geim AK, Novoselov KS: Control of graphene's properties by reversible hydrogenation: evidence for graphane. Science 2009, 323: 610. 10.1126/science.1167130View Article
                  11. Sofo JO, Chaudhari AS, Barber GD: Graphane: a two-dimensional hydrocarbon. Phys Rev B 2007, 75: 153401.View Article
                  12. Boukhvalov DW, Katsnelson MI: Chemical functionalization of graphene. J Phys Condens Matter 2009, 21: 344205. 10.1088/0953-8984/21/34/344205View Article
                  13. Ryu S, Han YM, Maultzsch J, Heninz TF, Kim P, Steigerwald ML, Brus LE: Reversible basal plane hydrogenation of graphene. Nano Lett 2008, 8: 4597. 10.1021/nl802940sView Article
                  14. Balog R, JØrgensen B, Nilsson L, Andersen M, Rienks E, Bianchi M, Fanetti M, Lægsgaard E, Baraldi A, Lizzit S, Sljivancanin Z, Besenbacher F, Hammer B, Pedersen TG, Hofmann P, Hornekær L: Bandgap opening in graphene induced by patterned hydrogen adsorption. Nat Mater 2010, 9: 315. 10.1038/nmat2710View Article
                  15. Dikin DA, Stankovich S, Zimney EJ, Piner RD, Dommett GHB, Evmenenko G, Nguyen ST, Ruoff RS: Preparation and characterization of graphene oxide paper. Nature 2007, 448: 457. 10.1038/nature06016View Article
                  16. Park S, Ruoff RS: Chemical methods for the production of graphenes. Nat Nanotechnol 2009, 4: 217. 10.1038/nnano.2009.58View Article
                  17. Chuang C, Puddy RK, Lin H-D, Lo S-T, Chen T-M, Smith CG, Liang C-T: Experimental evidence for efros-shklovskii variable range hopping in hydrogenated graphene. Solid State Commun 2012, 152: 905. 10.1016/j.ssc.2012.02.002View Article
                  18. Boukhvalov DW, Katsnelson MI, Lichtenstein AI: Hydrogen on graphene: electronic structure, total energy, structural distortions and magnetism from first-principles calculations. Phys Rev B 2008, 77: 035427.View Article
                  19. Withers F, Russo S, Dubois M, Craciun MF: Tuning the electronic transport properties of graphene through functionalisation with fluorine. Nano Res Letts 2011, 6: 526. 10.1186/1556-276X-6-526View Article
                  20. Singh AK, Penev ES, Yakobson BI: Vacancy clusters in graphane as quantum dots. ACS Nano 2010, 4: 3510. 10.1021/nn1006072View Article
                  21. Luth H: Surface and Interfaces of Solid Materials. New York: Springer Press; 1995.View Article
                  22. Suzuki M, Ishibashi M, Ida T, Aoyagi Y: Quantum dot formation in single-wall carbon nanotubes. Jpn J Appl Phys 1915, 2001: 40.
                  23. McEuen PL, Bockrath M, Cobden DH, Yoon Y-G, Louie SG: Disorder, pseudospins, and backscattering in carbon nanotubes. Phys Rev Lett 1999, 83: 5098. 10.1103/PhysRevLett.83.5098View Article
                  24. Zhou C, Kong J, Yenilmez E, Dai H: Modulated chemical doping of individual carbon nanotubes. Science 2000, 290: 1552.View Article
                  25. Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, Grigorieva IV, Firsov AA: Electric field effect in atomically thin carbon films. Science 2004, 306: 666. 10.1126/science.1102896View Article
                  26. Luo Z, Yu T, Kim K-J, Ni Z, You Y, Lim S, Shen Z, Wang S, Lin J: Thickness-dependent reversible hydrogenation of graphene layers. ACS Nano 2009, 3: 1781. 10.1021/nn900371tView Article
                  27. Ponomarenko LA, Yang R, Mohiuddin TM, Katsnelson MI, Novoselov KS, Morozov SV, Zhukov AA, Schedin F, Hill EW, Geim AK: Effect of a high-κ environment on charge carrier mobility in graphene. Phys Rev Lett 2009, 102: 206603.View Article
                  28. Connolly MR, Chiou KL, Smith CG, Anderson D, Jones GAC, Lombardo A, Fasoli A, Ferrari AC: Scanning gate microscopy of current-annealed single layer graphene. Appl Phys Lett 2010, 96: 113501. 10.1063/1.3327829View Article
                  29. Ponomarenko LA, Schedin F, Katsnelson MI, Yang R, Hill EW, Novoselov KS, Geim AK: Chaotic dirac billiard in graphene quantum dots. Science 2008, 320: 356. 10.1126/science.1154663View Article
                  30. Nazarov YV, Blanter YM: Quantum Transport Introduction to Nanoscience. Cambridge: Cambridge University Press; 2009.View Article
                  31. Ruzin IM, Chandrasekhar V, Levin EI, Glazman LI: Stochastic Coulomb blockade in a double-dot system. Phys Rev B 1992, 45: 13469. 10.1103/PhysRevB.45.13469View Article
                  32. Kemerink M, Molenkamp LW: Stochastic Coulomb blockade in a double quantum dot. Appl Phys Lett 1994, 65: 1012. 10.1063/1.112209View Article
                  33. Ishibashi K, Suzuki M, Ida T, Aoyagi Y: Formation of coupled quantum dots in single-wall carbon nanotubes. Appl Phys Lett 1864, 2001: 79.
                  34. Notargiacomo A, Gaspare LD, Scappucci G, Mariottini G, Evangelisti F, Giovine E, Leoni R: Single-electron transistor based on modulation-doped SiGe heterostructures. Appl Phys Lett 2003, 83: 302. 10.1063/1.1592883View Article
                  35. Guttinger J, Stampfer C, Fery T, Ihn T, Ensslin K: Transport through a strongly coupled graphene quantum dot in perpendicular magnetic field. Nano Res Letts 2011, 6: 253.View Article
                  36. Liu XL, Hug D, Vandersypen MK: Gate-defined graphene double quantum dot and excited state spectroscopy. Nano Letts 2010, 10: 1623. 10.1021/nl9040912View Article
                  37. Molitor F, Dröscher S, Güttinger J, Jacobsen A, Stamper C, Ihn T, Ensslin K: Transport through graphene double dots. Appl Phys Letts 2009, 94: 222107. 10.1063/1.3148367View Article
                  38. Ishigami M, Chen JH, Cullen WG, Fuhrer MS, Williams EW: Atomic structure of graphene on SiO2. Nano Lett 2007, 7: 1643. 10.1021/nl070613aView Article
                  39. Schedin F, Geim AK, Morozov SV, Hill EW, Blake P, Katsnelson MI, Novoselov KS: Detection of individual gas molecules adsorbed on graphene. Nat Mate 2007, 6: 652. 10.1038/nmat1967View Article
                  40. Hashimoto A, Suenage K, Gloter A, Urita K, Iijima S: Direct evidence for atomic defects in graphene layers. Nature 2004, 430: 870. 10.1038/nature02817View Article
                  41. Joung D, Zhai L, Khondaker SI: Coulomb blockade and hopping conduction in graphene quantum dots array. Phys Rev B 2011, 83: 115323.View Article
                  42. Nielsen MA, Chuang IL: Quantum Computation and Quantum Information. Cambridge: Cambridge University Press; 2000.
                  43. Loss D, DiVincenzo DP: Quantum computation with quantum dots. Phys Rev A 1998, 57: 120. 10.1103/PhysRevA.57.120View Article


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