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Graphene is a single atomic layer of carbon arranged in a honeycomb structure. This two-dimensional, zero-bandgap semimetal possesses a unique linear band structure [1] that gives it exceptionally high carrier mobilities [2, 3] and thermal conductivities [4].

Experimental studies have shown that the supporting substrate plays a critical role in perturbing and determining the electronic and topographic properties of graphene [2, 5-12].

The most commonly used SiO₂/Si supporting substrates induce spatial doping fluctuations, or charge inhomogeneity, in the graphene independent of the topographic fluctuations [13].

While graphene can be grown epitaxially on SiC(0001), the surface steps lead to carrier scattering that depends on the height and type of step [14]. Recent work studying graphene on atomically flat substrates has shown that hexagonal boron nitride greatly improves carrier mobility and reduces charge inhomogeneity in the graphene [12].

Understanding the electronic and topographic properties of graphene on the atomic scale is necessary to successful integration of graphene with conventional substrates. Due to its ubiquity in electronic devices, silicon is the primary substrate of technological interest.

The removal of hydrogen from the interface of graphene and the Si(100) – 2×1:H surface (leaving clean Si(100) – 2×1) leads to bond formation between the graphene and the Si(100) – 2×1 surface. Motivated by the potential for novel interactions with a more reactive substrate, due to its high density of dangling bonds, we report a study of nanoscale graphene flakes deposited on the clean Si(111) – 7×7 surface, a well-known surface reconstruction [15-17] of a commonly used Si facet.

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While the most graphene studies use insulating supporting substrates, a recent study fabricated graphene field-effect transistors on Si(111) and demonstrated high carrier mobilities [18]. Simulations also suggest that particular orientations of graphene on Si(111) could improve photovoltaic devices [19]. Recent work has examined devices based on the Schottky barrier formed at the interface between graphene and hydrogen-passivated Si(111) [20-22]. The band alignment on which these devices are based is very sensitive to the local atomic structure of the Si(111). The graphene on Si(111) – 7×7 system has been studied over large areas by atomic force microscopy (AFM), ex-site Raman spectroscopy, and Kelvin probe measurements [9]. However, there are no studies of graphene on Si(111) with atomic scale resolution, and determination of the type of bonding formed at the interface between the graphene and the Si(111) are inconclusive [9].

Motivated by the crucial importance of the graphene-substrate interactions and open questions about graphene on Si(111), studying the interface between graphene and Si(111) with atomic precision is necessary. We utilize ultrahigh vacuum scanning tunneling microscopy (UHV-STM), scanning tunneling spectroscopy (STS), and in situ graphene deposition to study the interactions between graphene and the Si(111) – 7×7 reconstructed surface [15-17, 23]. Due to the high density of dangling bonds of the Si(111) – 7×7 surface, 19 per unit cell [15], we expect that it will be very reactive and a very perturbative substrate for graphene. Combining these experiments with density functional theory calculations (DFT) helps us determine the chemistry and mechanical stability of the interface.

76 3.2 Methods

3.2.1 Scanning Tunneling Microscopy and Preparation of Si(111) – 7×7 Samples These experiments were performed on a homebuilt UHV-STM with a base pressure of 2.5×10^-11 Torr [24] operated at room temperature using electrochemically etched tungsten tips. We probed the local density of states (LDOS) of the sample using constant-spacing scanning tunneling spectroscopy (STS) in which the tip feedback is turned off at predetermined locations and the tip–sample bias swept through a specified range while recording the tunneling current. The tunneling conductance was calculated from the acquired (I – V) data. In our system, the tip is grounded through a current amplifier, and the bias is applied to the sample. In this setup, a filled states STM image indicates that a negative bias was applied to the sample. Conversely, an empty states STM image indicates that a positive bias was applied to the sample.

The Si(111) substrates used for this work were B-doped with resistivities between 5 mΩ-cm and 35 mΩ-cm. Prior to preparation of the 7×7 surface reconstruction [15], all samples were degassed by Joule heating at 600 °C – 700 °C in UHV, typically for 12 hours.

The sample surfaces were prepared by direct current, Joule heating to a sample temperature between 1200 and 1250 °C for 30 s. This process was repeated three times, while only holding the sample at temperature for 10 s during the last iteration. The samples were slowly cooled through the transition from the disordered 1×1 phase to the 7×7 surface reconstruction. The samples were examined with the STM prior to graphene deposition to ensure that the sample preparation was successful. Figure 3.1 shows typical empty (Figure 3.1a) and filled (Figure 3.1b) states STM topographs of samples after surface preparation.

The resolution of the Si(111) – 7×7 surface differs slightly between empty (positive bias

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here) and filled (negative bias) states. Both images also show adatom vacancies from the 7×7 reconstruction and spurious particulate contamination.

3.2.2 Dry Contact Transfer Method

In order to deposit nanometer-sized flakes of graphene without contaminating the highly reactive Si(111) – 7×7 surface, we use the dry contact transfer (DCT) method developed by Albrecht and Lyding [25] for carbon nanotube in UHV. We follow the development by Ritter and Lyding [5] and He et al. [6] for graphene deposition in UHV. In brief, a braided, fiberglass sheath is loaded with graphene flakes by rubbing against graphite powder or a highly ordered pyrolytic graphite crystal until the applicator is visibly gray. This step can occur either ex situ or in situ, if a graphite sample is prepared and loaded into the UHV system. Following a degassing step in UHV for 12 – 24 hours, the applicator is pressed gently into contact with the sample surface. The last step is repeated as necessary until graphene features can be located on the sample. Figure 3.2 shows an illustration of the applicator in relation to the target sample in an STM sample holder.

While use of this method for depositing graphene on the Si(100) – 2×1:H surface tended to give mostly graphene monolayer features [5], that has not been the case for the Si(111) – 7×7 surface. Most of the deposition on this surface tends to leave graphite clumps.

Many of the graphene monolayer features were attached to larger stacks of graphene multilayers or graphite. While this might be attributable to the increased reactivity of the clean Si(111) – 7×7 surface compared to the hydrogen-passivated Si(100) surface, variations in the size distribution of the graphite powder before loading the DCT applicator could also account for this difference. The DCT method also occasionally damages the Si(111) – 7×7 surface reconstruction by scratching the surface with the applicator. Applying too much shear

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force during the applicator-substrate contact step can account for the variability in observed surface damage.