Tuning the Electronic Structure of Graphite Moiré with Chromium

Article pubs.acs.org/JPCC

Tuning the Electronic Structure of Graphite Moiré with Chromium Deposition: a Scanning Tunneling Microscopy and Scanning Tunneling Spectroscopy Study Xin Zhang* and Hong Luo Department of Physics, University at Buffalo, the State University of New York, Buffalo, New York 14260, United States ABSTRACT: The growth of chromium (Cr) on moiré patterns (MPs) on highly oriented pyrolytic graphite (HOPG) surface was studied by scanning tunneling microscopy/ spectroscopy (STM/STS). At low coverage, Cr clusters can nucleate at top, bridge and hollow sites of the MP with no long-range or short-range orders. With increasing coverage, the size distribution of the Cr clusters remains constant and narrow, only the population increases. Compared to the situation before Cr deposition, the energy difference (ΔEVHS) between the two van Hove singularities (VHSs) of the twisted HOPG surface layer is enlarged while remaining linearly proportional to the sine of the twisting angle. The distance-dependence of the ΔEVHS enlargement, revealed by STS at extremely low Cr coverage, provides an estimate of each individual cluster’s ability of modifying the graphite’s electronic structure.

■

INTRODUCTION In recent years, the moiré pattern (MP) of monolayer graphene on metal or silicon carbide substrates has been widely used as a template to facilitate the growth of ordered arrays of monodisperse (equally sized) metal clusters1−4 or even adatom monomers.5−8 Such a system shows great application potential for its unique electronic, spintronic, optical, and catalytic properties.9,10 On the other hand, twisted graphene layers, which also present MP without involving a substrate of different material, have drawn a lot of attention11 because of their intriguing properties, such as twisting-angle-dependent van Hove singularities (VHSs),12−15 coexistence of massive and massless Dirac fermions,16 and the presence of superlattice Dirac points.17 However, little effort has been made to study the growth of metal nanostructures on twisted graphene layers. More importantly, how the presence of metal adatoms tunes the properties of the twisted graphene layers remains an area to be explored. In this paper, chromium (Cr) was deposited on the MP formed by the twisting of the surface layer of highly oriented pyrolytic graphite (HOPG) and the system was studied by scanning tunneling microscopy/spectroscopy (STM/STS). The Cr clusters show no preferential site-specific nucleation on the MP. The modification of the graphite electronic structure by the Cr clusters is revealed by STS as the energy difference (ΔEVHS) between the two VHS peaks is enlarged. The values of the ΔEVHS, both before and after the Cr deposition, are linearly proportional to the sine of the twisting angle. From the enlargement of ΔEVHS upon Cr deposition, an increase of the graphite’s Fermi velocity from 1.17 × 106 m/s to 1.41 × 106 m/s and a decrease of the interlayer hopping parameter from 0.068 to 0.050 eV can be determined. The spatial dependence of this effect around each individual cluster © 2014 American Chemical Society

is studied with extremely low Cr coverage, so that Cr clusters can be considered effectively isolated.

■

EXPERIMENTAL DETAILS HOPG surface was prepared by mechanical exfoliation. All the STM and STS measurements were carried out at room temperature (RT) with an Omicron ultrahigh vacuum STM system. STS measurements were done in the current imaging tunneling spectroscopy (CITS) mode with standard lock-in technique, by which an AC modulation is applied on top of the DC bias between the tip to surface and the signal is measured with a lock-in amplifier at the modulation frequency. When an MP surface was chosen for the Cr deposition study, the STM tip would be retracted and Cr was deposited in situ with an Omicron E-Beam evaporator. During the deposition, the sample was kept at RT. The deposition rate is 0.1 ML per minute. After the deposition, the tip would be reapproached to the surface and the original MP area can be found, thus, the topographic and electronic properties of the same area before and after the Cr deposition can be directly compared. The amount of Cr atoms deposited is estimated by calculating the sizes and density of the clusters on the surface, then, the coverage is determined by converting the atoms per cm2 to ML (1 ML = 1.9 × 1015 atoms/cm2, which equals to the density of unit cell in graphene18). For the surfaces shown in Figure 1 and Figure 2, the uncertainty of the coverage estimation is about ±0.0025 ML. Gaussian fittings of the data points were performed. Received: July 1, 2014 Revised: August 3, 2014 Published: August 13, 2014 20461

dx.doi.org/10.1021/jp506558j | J. Phys. Chem. C 2014, 118, 20461−20465

The Journal of Physical Chemistry C

Article

nucleation can be observed. The nucleation of Cr cluster can take place on hollow site, bridge site or atop site, as shown in Figure 1b−d. Such random nucleation happened for all the four MP surfaces measured in this study. The periodicities (D) of the four MPs are 3.3 nm, 4.8 nm, 6.5 and 6.7 nm. In comparison, when graphene with MPs grown on metal surfaces were used as substrate, metal clusters (Pt, Ir, and Ni) always nucleate on hollow site. Some even show a preference between fcc hollow site and hcp hollow site.1−4 Such difference may result from the different interaction with the substrates under the graphene layer, metal vs bulk graphite in this comparison. The bonding between the Cr clusters and the graphite substrate is found to be weak as the cluster can be moved by the STM tip occasionally during scanning. The graphite atomic lattice and the Moiré superlattice structure are intact under the clusters. In order to study the growth scenario of Cr clusters, STM images of the same MP area, with increasing Cr coverage (0.04, 0.05, and 0.07 ML), are shown in Figure 2a−c. The top terrace in the images presents a MP with D = 4.8 nm, as shown in the inset. The size distributions of the clusters in this MP area for all three coverages are shown in Figure 2d−f. The full widths at half-maximum of the distribution curves are almost constant for the increasing Cr coverage. Only the total population of the cluster increases. The electronic structure of the MP surfaces, both before and after the Cr deposition, was studied with STS in the CITS mode. During a CITS measurement, a tunneling spectrum is taken at each pixel of the scanning frame. The most significant feature in the differential conductance (dI/dV) curve of a graphite MP surface, shown in Figure 3a−c, is the presence of two peaks flanking the Fermi level, which represent the two VHS peaks in the density of states (DOS).12−15 Moreover, the states associated with the VHSs spatially concentrate in the bright areas of the MP, results in a spatial modulation of DOS at the energies close to the VHSs. This modulation is clearly revealed by comparing the topographic image of a MP with its dI/dV map at 0.16 V, close to the location of one VHS peak in energy scale, shown in the insets of Figure 3c. On the other hand, no modulation can be seen on the dI/dV map at −0.65 V, away from any VHS peak. Because the DOS modulation only exists at the energies of the VHSs, the averaged dI/dV curve taken from the MP’s bright areas and the averaged dI/dV curve from the MP’s dark areas only show amplitude difference around the VHS peaks, as can be seen in Figure 3a−c. Therefore, if we plot this difference as a function of the bias voltage, two peaks should be seen, which correspond to the exact energies of the VHSs. The difference between the dI/dV curves show in Figure 3a−c are plotted in Figure 3d−f, respectively. Two pounced peaks can be seen in each difference curve, marking the VHSs’ energies. For all the MPs studied, the energy difference between the two VHS peaks, ΔEVHS, increased after Cr deposition. For the case that a graphene layer twisted in orientation with respect to the underlying graphene12−14 or graphite,12,15 the value of ΔEVHS is linearly related to the sine of the twisting angle θ and the relation can be described by the equation:

Figure 1. (a) STM image of MP surface with 0.07 ML Cr coverage. Taken with I = 0.2 nA, V = −0.8 V. (b) I = 0.2 nA, V = −0.7 V, a Cr cluster nucleated at hollow site of the MP superlattice. (c) I = 0.4 nA, V = 1.0 V, a Cr cluster nucleated at bridge site. (d) I = 0.4 nA, V = 1.0 V, a Cr cluster nucleated at atop site. (f): the height profile alone the green line in part a, presenting the typical diameters and heights of the Cr clusters. The yellow scale bars (a−d) equal to 5 nm.

Figure 2. (a−c) Same MP area with increasing Cr coverage, the yellow scale bars equal to 30 nm. Key: (a) 0.04 ML, I = 0.1 nA, V = −0.8 V; (b) 0.05 ML, I = 0.2 nA, V = −0.75 V; (c) 0.07 ML, I = 0.15 nA, V = −0.8 V. Inset: I = 0.4 nA, V = 0.92 V, higher resolution image of the MP surface. (d−f) Corresponding size distributions of the Cr clusters on the MP area shown in parts a−c, respectively. Red curves are Gaussian fits of the data.

ΔEVHS = 2ℏvF K sin(θ /2) − 2tθ

■

(1)

Here vF is the Fermi velocity, K is the reciprocal lattice vector equals to 4π/3a (a = 0.246 nm, is the lattice constant for a single graphene layer), tθ is the interlayer hopping parameter. The value of ΔEVHS before and after the Cr deposition for all three MPs are plotted in Figure 4a as a function of sin(θ/2).

RESULTS AND DISCUSSION The STM topographic image of an MP area with nominally 0.07 ML Cr coverage is presented in Figure 1a. Cr atoms form small clusters on the MP surface. No order or site-specified 20462

dx.doi.org/10.1021/jp506558j | J. Phys. Chem. C 2014, 118, 20461−20465

The Journal of Physical Chemistry C

Article

Figure 3. (a−c) Differential conductance (dI/dV) curves taken over three MPs (D= 3.3, 4.8, and 6.5 nm) before and after Cr deposition. The blue (yellow) curves are the average of all dI/dV curves taken on the MP’s bright (dark) area. Inset: 13 × 12 nm, the topographic image and the differential conductance (dI/dV) maps at −0.65 and 0.16 V of the same MP area. The topographic image and the dI/dV maps were acquired simultaneously within one CIST measurement, from which the dI/dV curves (with Cr) shown in part c were extracted. (d−f) Differences between the blue and yellow curves in parts a−c plotted as a function of the bias voltage. Black curves are the raw data and the red curves are the Gaussian fits.

Figure 4. (a) ΔEVHS plotted as a function of sin(θ/2). The straight lines are the linear fits of the data points. (b) ΔEVHS plotted as a function of distance from the Cr cluster. Three sets of data and their Gaussian fits are displayed with different colors. The error bars represent the standard deviations in each case. The horizontal black line represents the value of ΔEVHS (0.22 eV) of the studied MP before Cr deposition.

which effectively changes the Fermi velocity and modifies the electronic structure of the twisted graphite layers. If this is the case, each Cr cluster should have its effective range. However, for the examples provided above, the value of ΔEVHS is found to be uniform over the MP, shows no distance-dependence. A possible reason for such observation is that the coverage of Cr is too high in these cases and the substrates are saturated for charge transfer. In order to verify this hypothesis and study the possible distance-dependent ΔEVHS, experiment was carried out with extremely low Cr coverage, ∼1.6 × 10−4 ML. At this coverage, the normal distance between neighboring clusters is larger than 100 nm. The values of ΔEVHS as a function of the distance from the clusters are shown in Figure 4b. For each data

Since each data point in Figure 4a is an average of many measurements, the error bar is within each symbol. The linear relation between the ΔEVHS and sin(θ/2) remains after deposition. The larger slope of the fit after deposition indicates an increased Fermi velocity. With the experiment data, the Fermi velocity is found from eq 1 to increase from 1.17 × 106 to 1.41 × 106 m/s while the interlayer hopping parameter decreased from 0.068 to 0.050 eV after the deposition. The latter is easy to understand as the interaction between the first and second graphite layer is weakened by the presence of Cr clusters on the other side of the surface graphite layer. The enlargement of ΔEVHS is attributed to the charge transfer19 between the Cr clusters and the graphite substrate, 20463

dx.doi.org/10.1021/jp506558j | J. Phys. Chem. C 2014, 118, 20461−20465

The Journal of Physical Chemistry C

■

point, the value of ΔEVHS was determined using the method described above for each MP unit cell, unit cell by unit cell away from a cluster. The value of ΔEVHS shows a distancedependence when there is enough separation between Cr clusters. The horizontal black line in Figure 4b represents the value of ΔEVHS before Cr deposition, ΔEVHS0, which is constant over the MP area. The largest value of ΔEVHS is always obtained at the location closest to the cluster, and decreases as the measurement location moves away from the cluster, eventually reaching a certain value, ΔEVHS′ that is very close to the value of ΔEVHS0 (cluster 3) or slightly larger (clusters 1 and 2). The variations of the ΔEVHS enlargement and its decreasing slope from cluster to cluster may be caused by two reasons. First, the existence of other clusters nearby but outside the imaged area will contribute to charge transfer. The second is the different sizes and geometric configurations of the surrounding clusters that will affect the result asymmetrically around the observed cluster. The size of the cluster will determine the amount of charger transfer and the specific geometric configuration between the cluster and the underlying protuberances of the MP may affect the spatial distribution of transferred charges. Nevertheless, all three curves shown in Figure 4b reach an value that is half way down to ΔEVHS′ at around 20−25 nm (3−4 MP unit cells) away from the cluster. This common feature of the space-dependent ΔEVHS, provides us an estimation of the effective distance of the effect of Cr clusters in modifying the graphite MP’s electronic structure. The sign and amount of charge transfer between the cluster and substrate can be determined by the equation19 q = πε0Δϕd 2/4l

Article

CONCLUSION Deposition of Cr on MP of HOPG form clusters distributed randomly over the surface, with no site-specific nucleation or order. The energy difference between the two VHSs of the graphite Moiré is enlarged by the presence of Cr clusters while its linear dependence on the twisting angle of the graphite surface layer remains. For the situations before and after Cr deposition, the surface’s Fermi velocity increased from 1.17 × 106 to 1.41 × 106 m/s, respectively, while the interlayer hopping parameter decreased respectively from 0.068 to 0.050 eV. The distance-dependence of the electronic structure modification caused by a Cr cluster was studied with extremely low Cr coverage. The amplitude of the ΔEVHS enlargement decreases as a function of the distance from the cluster and will reach its half way to ΔEVHS′ at about 20−25 nm away from the cluster.

■

AUTHOR INFORMATION

Corresponding Author

*(X.Z.) E-mail: xzhang37@buffalo.edu. Telephone: (716)-6456475. Fax: (716)-645-2507. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work was supported by NSF DMR1006286. REFERENCES

(1) N’Diaye, A. T.; Bleikamp, S.; Feibelman, P. J.; Michely, T. TwoDimensional Ir Cluster Lattice on a Graphene Moiré on Ir(111). Phys. Rev. Lett. 2006, 97, 215501−215504. (2) Pan, Y.; Gao, M.; Huang, L.; Liu, F.; Gao, H. J. Directed Selfassembly of Monodispersed Platinum Nanoclusters on Graphene Moiré Template. Appl. Phys. Lett. 2009, 95, 093106−093108. (3) Dooner, K.; Jakob, P. Structural Properties and Site Specific Interactions of Pt with the Graphene/Ru(0001) Moiré Overlayer. J. Chem. Phys. 2009, 131, 164701−164710. (4) Sicot, M.; Bouvron, S.; Zander, O.; Rüdiger, U.; Dedkov, Yu. S.; Fonin, M. Nucleation and Growth of Nickel Nanoclusters on Graphene Moiré on Rh(111). Appl. Phys. Lett. 2010, 96, 093115− 093117. (5) Gyamfi, M.; Eelbo, T.; Waśniowska, M.; Wiesendanger, R. Fe Adatoms on Graphene/Ru(0001): Adsorption Site and Local Electronic Properties. Phys. Rev. B 2011, 84, 113403−113406. (6) Gyamfi, M.; Eelbo, T.; Waśniowska, M.; Wehling, T. O.; Forti, S.; Starke, U.; Lichtenstein, A. I.; Katsnelson, M. I.; Wiesendanger, R. Orbital Selective Coupling Between Ni Adatoms and Graphene Dirac Electrons. Phys. Rev. B 2012, 85, 161406−161410(R). (7) Eelbo, T.; Waśniowska, M.; Thakur, P.; Gyamfi, M.; Sachs, B.; Wehling, T. O.; Forti, S.; Starke, U.; Tieg, C.; Lichtenstein, A. I.; Wiesendanger, R. Adatoms and Clusters of 3d Transition Metals on Graphene: Electronic and Magnetic Configurations. Phys. Rev. Lett. 2013, 110, 136804−136808. (8) Semidey-Flecha, L.; Teng, D.; Habenicht, B. F.; Sholl, D. S.; Xu, Y. Adsorption and Diffusion of the Rh and Au Adatom on Graphene moiré/Ru(0001). J. Chem. Phys. 2013, 138, 184710−184719. (9) Liu, X.; Wang, C. Z.; Hupalo, M.; Lin, H. Q.; Ho, K. M.; Tringides, M. C. Metals on Graphene: Interactions, Growth Morphology, and Thermal Stability. Crystals 2013, 3, 79−111. (10) Sarkar, S.; Moser, M. L.; Tian, X.; Zhang, X.; Al-Hadeethi, Y. F.; Haddon, R. C. Metals on Graphene and Carbon Nanotube Surfaces: From Mobile Atoms to Atomtronics to Bulk Metals to Clusters and Catalysts. Chem. Mater. 2014, 26, 184−195. (11) Hofmann, P. A Little Twist with Big Consequences. Nat. Mater. 2013, 12, 874−875.

(2)

Here ε0 is the dielectric constant, Δφ is the difference between the work functions of the cluster and the substrate, which is ∼0.3 eV in the case of Cr clusters on HOPG from the X-ray photoelectron spectroscopy study by V. D. Borman et al.,19 d is the diameter of the cluster and l = 0.3 nm is the separation between Cr layers. Therefore, a typical cluster found in our STM image with a diameter of 4.8 nm is negatively charged by q ≈ 1e (e is the electron charge). The presence of charge transfer is a common feature when the graphene is in contact with metals.20−23 Theoretical calculation21 and angle resolved photoemission spectroscopy (ARPES) studies22,23 have confirmed an offset of the graphene’s Dirac cone and an opening of a band gap at the Dirac point, as results of the charge transfer. The observed enlargement of the ΔEVHS here demonstrates the Dirac cone offset and band gap opening occurring in twisted graphene layers, thus providing another route to study the effect of charge transfer. As shown with the distance-dependent enlargement of ΔEVHS in Figure 4b, this new route provides a nanometer scale spatial resolution that cannot be matched by other techniques. Recently, Meng et al. demonstrated the decreasing of ΔEVHS and the enhancing of interlayer coupling by deposit single-molecule magnets onto the twisted graphene bilayer.24 However, such an effect was assigned to the adsorbent-induced alternation of graphene interlayer distance and no electronic or magnetic interaction between the adsorbent and the graphene was discussed. Such findings, together with the results in this report, suggest that different tuning of the graphene electronic structure can be achieved by carefully manipulating the different kinds of interactions between the adsorbents and graphene. 20464

dx.doi.org/10.1021/jp506558j | J. Phys. Chem. C 2014, 118, 20461−20465

The Journal of Physical Chemistry C

Article

(12) Li, G.; Luican, A.; Lopes dos Santos, J. M. B.; Castro Neto, A. H.; Reina, A.; Kong, J.; Andrei, E. Y. Observation of Van Hove Singularities in Twisted Graphene Layers. Nat. Phys. 2009, 6, 109− 113. (13) Yan, W.; Liu, M.; Dou, R.; Meng, L.; Feng, L.; Chu, Z.; Zhang, Y.; Liu, Z.; Nie, J.; He, L. Angle-Dependent Van Hove Singularities in a Slightly Twisted Graphene Bilayer. Phys. Rev. Lett. 2012, 109, 126801−126805. (14) Brihuega, I.; Mallet, P.; González-Herrero, H.; Trambly de Laissardière, G.; Ugeda, M. M.; Magaud, L.; Gómez-Rodríguez, J. M.; Ynduráin, F.; Veuillen, J.-Y. Unraveling the Intrinsic and Robust Nature of Van Hove Singularities in Twisted Bilayer Graphene by Scanning Tunneling Microscopy and Theoretical Analysis. Phys. Rev. Lett. 2012, 109, 196802−196806. (15) Zhang, X.; Luo, H. Scanning Tunneling Spectroscopy Studies of Angle-dependent Van Hove Singularities on Twisted Graphite Surface Layer. Appl. Phys. Lett. 2013, 103, 231602−231606. (16) Kim, K. S.; Walter, A. L.; Moreschini, L.; Seyller, T.; Horn, K.; Rotenberg, E.; Bostwick, A. Coexisting Massive and Massless Dirac Fermions in Symmetry-broken Bilayer Graphene. Nat. Mater. 2013, 12, 887−892. (17) Yan, W.; He, W. Y.; Chu, Z. D.; Liu, M.; Meng, L.; Dou, R. F.; Zhang, Y.; Liu, Z.; Nie, J. C.; He, L. Strain and Curvature Induced Evolution of Electronic Band Structures in Twisted Graphene Bilayer. Nature Commun. 2013, 4, 2159−2165. (18) Pi, K.; McCreart, K. M.; Bao, W.; Han, W.; Chiang, Y. F.; Li, Y.; Tsai, S. W.; Lau, C. N.; Kawakami, R. K. Electronic Doping and Scattering by Transition Metals on Graphene. Phys. Rev. B 2009, 80, 075406−075410. (19) Borman, V. D.; Pushkin, M. A.; Tronin, V. N.; Troyan, V. I. Evolution of the Electronic Properties of Transition Metal Nanoclusters on Graphite Surface. J. Exp. Theor. Phys. 2010, 110, 1005− 1025. (20) Liu, H.; Liu, Y.; Zhu, D. Chemical Doping of Graphene. J. Mater. Chem. 2011, 21, 3335−3345. (21) Giovannetti, G.; Khomyakov, P. A.; Brocks, G.; Karpan, V. M.; van den Brink, J.; Keely, P. J. Doping Graphene with Metal Contacts. Phys. Rev. Lett. 2008, 101, 026803−026806. (22) Varykhalov, A.; Scholz, M. R.; Kim, T. K.; Rader, O. Effect of Noble-metal Contacts on Doping and Band Gap of Graphene. Phys. Rev. B 2010, 82, 121101−121104(R). (23) Walter, A. L.; Nie, S.; Bostwick, A.; Kim, K. S.; Moreschini, L.; Chang, Y. J.; Innocenti, D.; Horn, K.; McCarty, K. F.; Rotenberg, E. Electronic Structure of Graphene on Single-crystal Copper Substrates. Phys. Rev. B 2011, 84, 195443−195449. (24) Meng, L.; Yan, W.; Yin, L.; Chu, Z. D.; Zhang, Y.; Feng, L.; Dou, R.; Nie, J. Tuning the Interlayer Coupling of the Twisted Bilayer Graphene by Molecular Adsorption. J. Phys. Chem. C 2014, 118, 6462−6466.

20465

dx.doi.org/10.1021/jp506558j | J. Phys. Chem. C 2014, 118, 20461−20465