Despite its extraordinary charge carrier mobility, the lack of an electronic bandgap in graphene limits its utilization in electronic devices. To overcome this issue, researchers have attempted to chemically modify the pristine graphene lattice in order to engineer its electronic bandstructure. While significant progress has been achieved, aggressive chemistries are often employed which are difficult to pattern and control. In an effort to overcome this issue, here we utilize the well-defined van der Waals interface between crystalline Ge(110) and epitaxial graphene to template covalent chemistry. In particular, by annealing atomically pristine graphene-germanium interfaces synthesized by chemical vapor deposition under ultra-high vacuum conditions, chemical bonding is driven between the germanium surface and the graphene lattice. The resulting bonds act as charge scattering centers that are identified by scanning tunneling microscopy. The generation of atomic-scale defects is independently confirmed by Raman spectroscopy, revealing significant densities within the graphene lattice. The resulting chemically modified graphene has the potential to impact next-generation nanoelectronic applications.

The electronic bandstructure of graphene possesses gapless, linear dispersion at the corners of the Brillouin zone in the vicinity of the Fermi level (Ef). The massless Dirac carriers within this energetic window around Ef exhibit exceptional carrier mobility that is of high interest for a variety of electronic applications.1 However, the most pervasive electronic technology, digital logic, requires the use of a semiconducting material.2 Thus, significant efforts have been devoted to the opening of an electronic bandgap within graphene.3 

Consistent with theoretical considerations, research has shown that one such approach for realizing a gapped graphene-based material involves one-dimensional quantum confinement of graphene to atomic length scales.4 While experimental realizations of graphene nanoribbons have indeed shown a width-dependent bandgap, these materials present several challenges that have hindered progress toward fully integrated electronic devices.5–8 Alternative approaches have attempted to physically modify the bonding geometry of graphene by attaching chemical moieties to the basal plane.9–16 The introduction of chemical bonds into the graphene lattice leads to physical modifications and corresponding electronic structure changes.17,18 In particular, density functional theory predicts that an electronic bandgap of 0.77 eV is achievable when approximately half of the carbon atoms in graphene are covalently reacted with hydrogen.19 

Experimental efforts to realize such chemical modifications have initially focused on the selection of appropriate chemical moieties to concurrently achieve thermodynamic stability and bandstructure modification. The highly stable chemical nature of graphene has necessitated the use of aggressive chemistries towards this end.20–22 For example, cracking the gaseous diatomic species H2, N2, and O2 under ultra-high vacuum (UHV) conditions has yielded success in generating highly local chemical modifications to the graphene lattice.23–27 The thermodynamic stability and chemical reversibility of these chemistries make them strong candidates for the development of a tunable, electronically gapped graphene monolayer. A remaining challenge for the field is to introduce the chemical modifications in a spatially periodic manner, which is expected to minimize scattering and thus preserves high mobility.28 This periodic control has proven to be a significant experimental challenge, as the chemical reactivity of the atomic species often precludes patterning or directed assembly schemes. Some success has been achieved by utilizing the Moiré pattern on Ir(111) to template graphene hydrogenation, leading to the opening of sizeable bandgaps as confirmed by angle-resolved photoelectron spectroscopy (ARPES),29 but work remains to characterize and exploit this system outside of UHV conditions.

Here, we present an alternative strategy that introduces covalent modification of the graphene lattice via an adjacent germanium crystal. Utilizing a previously described atmospheric-pressure chemical vapor deposition synthetic procedure to achieve atomically pristine graphene-germanium interfaces,30–32 we subsequently drive covalent chemical bonding with the underlying reconstructed Ge(110) surface to produce chemically modified graphene. This chemical modification is achieved by UHV annealing to temperatures approaching the melting point of bulk germanium [we use the dimensionless homologous temperature (TH) throughout this work, defined as TH = T/TM, where TM is the bulk melting point of germanium in K and T is the experimental temperature in K]. Atomic-resolution scanning tunneling microscopy (STM) is used to confirm and characterize the resulting modification of the graphene lattice. The scattering behavior of the defect centers is predominantly characteristic of isolated, single-site sp3 bonds introduced into the otherwise pristine graphene lattice. The increased intensity of the Raman D-band after high-temperature annealing further confirms the presence of point defects within the graphene atomic plane. The crystalline nature of the underlying Ge(110) surface allows the resulting chemical modification to occur in a spatially periodic manner, as is required for a variety of nanoelectronic applications.

Figure 1 shows a collection of STM images that illustrate the various surface reconstructions observed on Ge(110) crystals. Figure 1(a) depicts the native (without graphene) reconstruction of the Ge(110) surface after UHV sputter/annealing preparation. As seen in the fast Fourier transform (FFT) of the STM data, the ordering is defined primarily by a periodicity of ∼1.43 nm along the ⟨1–12⟩ crystallographic directions of the surface, resembling the Ge(110) (16 × 2) reconstructed surface.33 Figure 1(b) shows the morphology of the Ge(110) surface underneath graphene after the ex-situ growth procedure. The random in-plane surface morphology is characterized by nanometer-scale, isotropic protrusions with apparent heights less than 0.2 nm. After annealing to TH ≈ 0.66 (T =600 °C) for 20 min under UHV conditions, quasi-1D features begin to emerge [Fig. 1(c)]. The primary inter-row length scale is ∼1.72 nm, although the reconstruction has a variety of smaller, more subtle features.34 Finally, upon annealing to 0.85 ≤ TH < 1 for 20 min, a modified 1D symmetry is observed. The uncertainty in the temperature measurement is due to the modified emissivity of Ge near TM. This symmetry, more closely examined in Fig. 2, is characterized by a smaller inter-row periodicity of ∼0.56 nm. As reported previously,32,34 the reconstructed surfaces shown in Figs. 1(b) and 1(c) do not interact chemically with the graphene overlayer and are simply the result of germanium surface rearrangement. The reconstructed Ge(110) surface shown in Fig. 1(d), however, corresponds to the onset of chemical interactions between the materials, which is independently confirmed in the STM and Raman spectroscopy data in Figs. 3 and 4, respectively.

FIG. 1.

(a) STM image of a Ge(110) surface prepared by ion bombardment and annealing under UHV (V = 2 V, I = 200 pA, and scale bar = 8 nm). Inset: an atomic-scale view of the reconstruction. (b) STM image of the Ge(110) surface after graphene growth without annealing (V = 1.4 V, I = 80 pA, and scale bar = 2 nm). (c) STM image of the Ge reconstruction underneath graphene at TH ≈ 0.66 (V = 1.4 V, I = 80 pA, and scale bar = 2 nm). (d) STM image of the Ge reconstruction after annealing to TH > 0.85 (V = −0.8 V, I = 120 pA, and scale bar = 2 nm).

FIG. 1.

(a) STM image of a Ge(110) surface prepared by ion bombardment and annealing under UHV (V = 2 V, I = 200 pA, and scale bar = 8 nm). Inset: an atomic-scale view of the reconstruction. (b) STM image of the Ge(110) surface after graphene growth without annealing (V = 1.4 V, I = 80 pA, and scale bar = 2 nm). (c) STM image of the Ge reconstruction underneath graphene at TH ≈ 0.66 (V = 1.4 V, I = 80 pA, and scale bar = 2 nm). (d) STM image of the Ge reconstruction after annealing to TH > 0.85 (V = −0.8 V, I = 120 pA, and scale bar = 2 nm).

Close modal
FIG. 2.

(a) STM image of the Ge reconstruction observed after high temperature annealing underneath the graphene (V = −0.8 V, I = 120 pA, and scale bar = 2 nm). (b) Schematic illustrating symmetry and periodicity of the Ge(110) surface. (c) Line profile taken along the gray line in (a). (d) Line profile along the blue line in (a).

FIG. 2.

(a) STM image of the Ge reconstruction observed after high temperature annealing underneath the graphene (V = −0.8 V, I = 120 pA, and scale bar = 2 nm). (b) Schematic illustrating symmetry and periodicity of the Ge(110) surface. (c) Line profile taken along the gray line in (a). (d) Line profile along the blue line in (a).

Close modal
FIG. 3.

(a) STM topography of the reconstructed Ge(110) surface after high-temperature annealing with corresponding FFT in (b) (V = −0.8 V, I = 120 pA, and scale bar = 2 nm). (c) STM topography of the same region from (a) taken at a bias near Ef in order to image the overlying graphene. The corresponding FFT is provided in (d) (V = −0.1 V, I = 120 pA, and scale bar = 2 nm).

FIG. 3.

(a) STM topography of the reconstructed Ge(110) surface after high-temperature annealing with corresponding FFT in (b) (V = −0.8 V, I = 120 pA, and scale bar = 2 nm). (c) STM topography of the same region from (a) taken at a bias near Ef in order to image the overlying graphene. The corresponding FFT is provided in (d) (V = −0.1 V, I = 120 pA, and scale bar = 2 nm).

Close modal
FIG. 4.

(a) STM topography of the graphene lattice on top of the Ge(110) surface after high-temperature annealing with three-fold symmetric defects highlighted with green circles (V = 0.2 V, I = 120 pA, and scale bar = 4 nm). (b) STM of quasiparticle scattering in graphene due to atomic defects in the carbon lattice (V = 0.2 V, I = 120 pA, and scale bar = 1 nm). (c) Close-up STM image of a point defect in graphene exhibiting three-fold scattering symmetry (V = 0.2 V, I = 120 pA, and scale bar = 1 nm). (d) FFT of the STM image in (a) revealing directional scattering and quasiparticle chirality effects—the reciprocal lattice is highlighted in red, while the scattering intensity at the K/K′ points is highlighted in green.

FIG. 4.

(a) STM topography of the graphene lattice on top of the Ge(110) surface after high-temperature annealing with three-fold symmetric defects highlighted with green circles (V = 0.2 V, I = 120 pA, and scale bar = 4 nm). (b) STM of quasiparticle scattering in graphene due to atomic defects in the carbon lattice (V = 0.2 V, I = 120 pA, and scale bar = 1 nm). (c) Close-up STM image of a point defect in graphene exhibiting three-fold scattering symmetry (V = 0.2 V, I = 120 pA, and scale bar = 1 nm). (d) FFT of the STM image in (a) revealing directional scattering and quasiparticle chirality effects—the reciprocal lattice is highlighted in red, while the scattering intensity at the K/K′ points is highlighted in green.

Close modal

As the reconstruction in Fig. 1(d) is concomitant with the emergence of chemical interactions at the interface, its geometry is examined in detail in Fig. 2(a). The bias-dependent transparency of graphene to tunneling electrons35 accommodates a direct visualization of the underlying Ge surface [Fig. 2(a)]. The surface has two-fold symmetry, with the linear structures parallel to the blue line in Fig. 2(a) oriented along the [1–10] surface direction. Comparing the observed patterns to the theoretically unreconstructed Ge(110) surface [Fig. 2(b)], it is clear that the structures share similar symmetry groups. Notably, the Ge(110) surface can be characterized by two fundamental length scales: inter-row periodicity (aGe = 0.566 nm) along the [001] direction and intra-row periodicity [(√2/2) aGe = 0.380 nm] along the [1–10] direction. The intra-row periodicity that is highlighted with the green ovals in Figs. 2(a) and 2(b) is attributed to Ge diatomic pairs, similar to dimers on Si(100) surfaces. Comparing both the theoretical length scales in Fig. 2(b) to the experimental observations in Figs. 2(c) and 2(d), we see excellent agreement between the two sets of data, indicating that the Ge(110) surface likely possesses a morphology that is similar to the unreconstructed schematic in Fig. 2(b). Since this surface reconstruction has not previously been reported for the bare Ge(110) surface following UHV preparation, we conclude that the graphene overlayer is responsible for stabilizing this structural motif.

Further information about the interfacial symmetry can be obtained through the FFT of the STM images in Figs. 3(a) and 3(c). As indicated by the green circles in Fig. 3(b), the symmetry along the [001] surface direction is prominently visible, indicating a periodicity of ∼0.56 nm, again consistent with the unreconstructed Ge(110) surface along the [001] direction. The graphene lattice (orange circles) is not observed until the tunneling bias is brought close to Ef, as seen in Figs. 3(c) and 3(d). Interestingly, the Ge(110) surface remains visible at this low bias, but its structural features are obscured by the graphene. Atomic-scale defects are visible in the graphene lattice at −0.1 V [dark green circles in Fig. 3(c)] and demonstrate three-fold scattering symmetry characteristic of single carbon atom modifications.36 Of further note is the emergence of periodicities at the K/K′ points in reciprocal space [green circles in Fig. 3(d)], characteristic of high-momentum intervalley quasiparticle scattering. Intervalley quasiparticle scattering in graphene requires a defect capable of transferring enough momentum to the carrier to displace it from one corner of the Brillouin zone to another.37 Accommodating the inverse relationship between physical and momentum space, only atomic-scale defects possess enough momentum to achieve this process. Scattered carriers from defect sites then interact with non-scattered carriers, which leads to the emergence of quantum interference and the characteristic charge density patterns displayed in Figs. 4(b) and 4(c).

Figure 4(a) presents an atomically resolved STM image of the graphene lattice on the reconstructed Ge(110) surface. This image reveals key deviations from an intrinsic graphene lattice. First, the physical topography of the lattice is modified by the underlying germanium surface, showing both depressions and protrusions along the [001] direction associated with the underlying reconstruction. Second, atomic-scale modifications to the graphene lattice appear as isotropic perturbations confined to single carbon atoms within the graphene lattice. These are identified with white arrows in Fig. 4(b). Third, the emergence of three-fold symmetric scattering centers appears throughout the graphene lattice [green circles in Figs. 4(a)–4(c)], signaling the emergence of a localized disruption to the sp2 electronic hybridization, as shown in Refs. 36 and 37. It is noteworthy that while the first two effects (underlying reconstruction and point-like protrusions) can be observed upon lower temperature annealing (TH ≈ 0.66), the third modification is only present after annealing to TH > 0.85. The emergence of point scatterers within the graphene lattice signals the onset of covalent bonding between the graphene and underlying germanium.

Close inspection of the scattering patterns in Figs. 4(b) and 4(c) reveals unique attributes with respect to conventional point defects in graphene monolayers. For example, most of the scattering centers observed in Fig. 4(a) (green circles) possess three-fold symmetry due to the modification of the graphene sp2 bonding structure at a single point.36 As only a single sublattice is perturbed by this modification, pseudospin conservation limits scattering events to yield trigonal scattering patterns. The global scattering also exhibits anisotropic character. As seen in Fig. 4(d), the quasiparticle scattering is 2–4 times stronger orthogonal to the [001] surface direction [light green circles in Fig. 4(d)]. The three-fold symmetric point scattering at atomic defects cannot explain this deviation from ideality and is likely related to influences from the underlying substrate. Finally, high-resolution FFTs [Fig. 4(d)] further reveal the unique scattering behavior of the carriers in this chemically modified graphene system. The observation of semicircles instead of full circles at the K/K′ points [inset, Fig. 4(d)] shows that the carrier pseudospin plays a significant role in the scattering processes and modifies conventional mechanisms based purely on carrier momentum.38 

Finally, the introduction of chemical bonding into the graphene system is readily identifiable via vibrational Raman spectroscopy because of the scattering modes supported by defects in the atomic lattice. The graphene D-band, which is observed due to a single-resonance process where a defect and phonon mediate the scattering, is the most common metric to quantify lattice defects. As seen in Fig. 5(a), the Raman spectrum for graphene grown on Ge(110) and annealed to TH ≈ 0.66 in UHV shows a minor defect-related scattering peak. This result is similar to the sample immediately following CVD growth. A histogram of the D-peak area divided by the G-peak area [Fig. 5(c)] for spectra taken at various points across the surface shows that the average value is between 0.1 and 0.2, which is the characteristic of high-quality CVD graphene. In contrast, after annealing to TH > 0.85, the spectra possess starkly different characteristics such as the prominent emergence of both D and D′ bands related to the introduction of point defects. Again, looking at the ratio between the D and G peak areas, the spatial variation reveals a notable increase in the D/G ratio. With positive values ranging between 0.5 and 6, annealing introduces a significant quantity of defect scattering centers into graphene. Compared to Raman data from ion bombardment studies of graphene, the average defect separation should be less than 10 nm.39 This observation is in good agreement with the STM data, thus providing a lower bound on the defect density.

FIG. 5.

Representative Raman spectrum for graphene on Ge(110) before (a) and after (b) high temperature annealing in UHV. (c) Histogram of the D/G peak ratio for spatially resolved spectra across the graphene/Ge(110) surface. (d) Histogram of the D/G peak ratio for the corresponding surface after high-temperature annealing.

FIG. 5.

Representative Raman spectrum for graphene on Ge(110) before (a) and after (b) high temperature annealing in UHV. (c) Histogram of the D/G peak ratio for spatially resolved spectra across the graphene/Ge(110) surface. (d) Histogram of the D/G peak ratio for the corresponding surface after high-temperature annealing.

Close modal

In combination with the Raman spectroscopy results, the STM data bear out the chemical modification of the graphene lattice after UHV annealing at TH > 0.85. However, the synthesis of the graphene itself occurs at TH > 0.85. It is clear from previous work32,34 and the Raman spectra in Figs. 5(a) and 5(c) that the graphene lattice displays minimal defect density after the ex-situ growth procedure and with lower temperature (TH ≈ 0.66) annealing. To understand the differences between the two high temperature (TH > 0.85) annealing procedures, we examine the environment of the graphene/germanium systems during both processes. During the growth, heating is done under atmospheric pressures of CH4 and H2. As addressed in previous work,31 the presence of H2 serves to passivate the Ge surface during the growth procedure. However, as the graphene/germanium system is annealed to TH ≈ 0.66 in UHV, the hydrogen is removed and the germanium surface reconstructs underneath.34 It thus appears likely that the formation of chemical bonds between the graphene and germanium occurs at elevated temperatures (TH > 0.85) in UHV due to the presence of the unpassivated germanium surface.

In conclusion, graphene lattices are chemically modified by driving covalent bonding with underlying Ge substrates. The bonding is achieved by high-temperature annealing procedures under UHV to produce chemically homogeneous, single sp3 bonds within the graphene lattice. During the annealing procedure, the Ge(110) surface reconstructs to a symmetry approaching the unreconstructed (110) surface. Finally, Raman spectroscopy confirms the emergence of the defect centers in the graphene lattice with characteristic length scales consistent with STM observations. The introduction of point defects into the graphene lattice on an ordered, semiconductor surface marks a significant development toward the realization of tunable electronic properties in graphene monolayers with potential utility in electronic applications.

This work was performed, in part, at the Center for Nanoscale Materials, a U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences User Facility under Contract No. DE-AC02-06CH11357. In addition, this work was supported by the Office of Naval Research (Grant No. N00014-17-1-2993) and the National Science Foundation Graduate Fellowship Program (DGE-1324585 and DGE-0824162). R.M.J. and M.S.A. acknowledge the support by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award No. DE-SC0016007 for the graphene synthesis, and R.M.J. also acknowledges the support from the Department of Defense (DOD) Air Force Office of Scientific Research through the National Defense Science and Engineering Graduate Fellowship Program (No. 32 CFR 168a).

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