Abstract
When two-dimensional (2D) atomic crystals are brought into close proximity to form a van der Waals heterostructure, neighbouring crystals may influence each other’s properties. Of particular interest is when the two crystals closely match and a moiré pattern forms, resulting in modified electronic and excitonic spectra, crystal reconstruction, and more. Thus, moiré patterns are a viable tool for controlling the properties of 2D materials. However, the difference in periodicity of the two crystals limits the reconstruction and, thus, is a barrier to the low-energy regime. Here, we present a route to spectrum reconstruction at all energies. By using graphene which is aligned to two hexagonal boron nitride layers, one can make electrons scatter in the differential moiré pattern which results in spectral changes at arbitrarily low energies. Further, we demonstrate that the strength of this potential relies crucially on the atomic reconstruction of graphene within the differential moiré super cell.
INTRODUCTION
Van der Waals heterostructures allow combining different two-dimensional (2D) materials into functional stacks (1, 2), which has already produced a range of interesting electronic (3, 4) and optoelectronic (5–8) devices and resulted in observation of exciting physical phenomena. The large variety of the heterostructures is mainly due to the large selection of 2D materials. However, the assembly of van der Waals heterostructures allows one extra degree of freedom: Apart from the selection of the sequence of the 2D crystals, the individual crystals can be differently oriented with respect to each other. Previously, such control over the rotational alignment between crystals resulted in the observation of the resonant tunneling (9–11), renormalization of exciton binding energy (12), and insulating (13) and superconducting (4) states.
Probably one of the most spectacular results of the rotational alignment between different 2D crystals is the observation of the band reconstruction due to electron scattering on the moiré pattern in graphene aligned with hexagonal boron nitride (hBN). Because the lattice constants of graphene and hBN are relatively close to each other, the alignment between the two crystals leads to the formation of a moiré pattern (14, 15) with a relatively small wave vector, which results in the appearance of the secondary Dirac points (16–18) in the electronic spectrum. Furthermore, the strong van der Waals interaction also leads to the atomic reconstruction of the graphene lattice (19–22). Unfortunately, the characteristic energies at which the electronic spectrum can be reconstructed are given by the difference between the lattice constants of graphene and hBN, which does not allow changes to be made to the low-energy part of the spectrum.
Here, we demonstrate how we can gain further control over the band reconstruction of graphene using the differential between two moiré patterns (super-moiré) created by top and bottom hBN in hBN/graphene/hBN heterostructures. Such super-moiré patterns are not related to the difference in the lattice constants between the two crystals and thus can be of any arbitrary wave number, which makes it possible to arrange the spectrum reconstruction at arbitrary low energies.
RESULTS
To this end, we created encapsulated graphene devices where the graphene layer was aligned to both bottom and top hBN layers (alignment angles θα and θβ). The fabrication and transfer procedures have been previously described in (23) with the exception that not only the bottom but also the top hBN is now crystallographically aligned to the graphene. Briefly, we started by identifying the top hBN layer on SiO2. We then use a thin film of polypropylene carbonate (PPC) on Polydimethylsiloxane (PDMS) to lift the hBN from its substrate. This film then facilitates bringing the top hBN into contact with a graphene crystal (Fig. 1, A and B). We use very long and straight edges of the crystals to identify crystallographic axes and align them using a commercially available transfer rig (24). The graphene can then be lifted away from its substrate (Fig. 1B). At this point, we perform atomic force microscopy (AFM) (14, 15, 19) and Raman spectroscopy (25) experiments on the hBN/graphene bilayer to confirm the alignment. One such AFM image is presented in Fig. 1E, showing the characteristic hexagonal pattern (the Fourier transformation is shown in Fig. 1H). The crystals are then aligned and brought into contact with a second thin hBN layer (typically less than 1.5 nm or five atomic layers thick; Fig. 1B). This layer is also lifted away from its substrate, leaving a triple layer on the thin polymer film (Fig. 1C). We then perform AFM and Raman characterization again. Figure 1F and Fig. 1I are an example of one of our double-aligned moiré AFM images and its Fourier transformation for the case of the second hBN layer being one atomic layer thick, which allows one to see both moiré patterns simultaneously. Although not immediately clear in the real-space image, the Fourier transformation shows two sets of peaks corresponding to two hexagonal patterns (red and green dashed hexagons), as also schematically shown in Fig. 1D. The triple layer is then misaligned (~15°) and placed on top of a thick substrate hBN (Fig. 1C). Last, we use standard lithographic techniques to create the Hall bar geometry.
(A) Step 1: A thick hBN layer is aligned and used to pick up graphene (Gr). (B) Step 2: A thin hBN layer is aligned and picked up forming a triple layer. (C) Step 3: The heterostructure is placed on top of a thick hBN layer at 15° rotation angle (the substrate). (D) Illustration of the two individual moiré patterns and super-moiré pattern for three overlapping hexagonal lattices. (E) AFM image of the moiré pattern after the graphene is picked up. This shows only one moiré periodicity. Scale bar, 100 nm [(F) shares this scale]. (F) AFM image of the moiré patterns after the second thin hBN layer is picked up. Here, periodicities due to both hBN layers are visible. (G) Reciprocal space image of graphene’s first Brillouin zone. G1 (blue),
The longitudinal resistance (Rxx) as a function of carrier concentration is shown in Fig. 2A. Here, apart from the resistance peak associated with the main Dirac point (26), several additional peaks can be seen. Most of such peaks correspond to the change of sign of the transversal (Hall) resistance (Rxy) measured in nonquantized magnetic field (Fig. 2B). Typically, if graphene is aligned with only one hBN, then a single moiré pattern is produced, and only one secondary Dirac point for electrons and one for holes can be seen at concentrations that correspond to the wave vector determined by the periodicity of the moiré pattern (14–18). Aligning graphene to both the top and the bottom hBN will produce two moiré patterns (if θα and θβ are not equivalent), which should result in two secondary Dirac points for electrons and two for holes. However, if electrons can feel potential from both moiré patterns simultaneously, then second-order processes can be allowed, which would result in the reconstruction of the electronic spectrum at many other wave vectors.
(A) Rxx as a function of n for one of our devices with
In quantized magnetic fields, Landau fans can be seen to originate from these peaks (Fig. 2C). The Landau fans for most peaks exhibit both positive and negative indices (positive and negative slopes in Fig. 2C), which suggests that those are originating from the additional Dirac points and not from the higher-order zone edges (27).
To interpret these additional peaks, we recall that the perfectly aligned graphene on hBN should produce moiré pattern with approximately 14-nm periodicity, which corresponds to carrier concentration n ≈ 2.3 × 1012 cm−2. Thus, for our double-aligned graphene, we interpret the most pronounced peaks at ±1.98 × 1012 and ±2.34 × 1012 cm−2 as coming from the moiré patterns from the top and bottom hBN layers.
The periodicity of a moiré pattern, L, can be related to the carrier concentration required to reach the edge of its first Brillouin zone by
Then, we would like to notice the small peaks at ±0.35 × 1012 cm−2, which corresponds to the largest differential moiré pattern. From the carrier concentration, we can infer a periodicity of 35 nm. Further still, there is a pronounced peak at approximately ±0.90 × 1012 cm−2, which would yield a period of 22.7 nm. These features represent previously impossible periodicities for the graphene/hBN moiré pattern.
In Fig. 3A, we schematically describe the geometric origin of the super-moiré features.
(A) Reciprocal-space image of the area around graphene’s K point.
To check that all these peaks originate from the spectrum reconstruction because of scattering on the additional periodic potential, we measured the Brown-Zak oscillations at elevated temperatures (T) where cyclotron oscillations are suppressed (Fig. 4). At T > 70 K, oscillations independent of the carrier concentration can be clearly seen. At low fields B < 2.5 T (Fig. 4B), the oscillations are periodic in 1/B with the fundamental field Bf = 9.3 T. Assuming a hexagonal unit cell, such fundamental field can be calculated to correspond to a moiré periodicity of 22.7 nm, which corresponds to the peak in Rxx at n = ±0.90 × 1012 cm−2 (Fig. 2A).
(A) Map of σxx(n, B); scale of blue to red is 0.5 to 70 e2/hour. (B) Zoom-in view of the low-field part of the map, marked by the yellow dashed rectangle in (A); scale of black to white is 7 to 37 e2/hour. The Brown-Zak oscillations correspond to a moiré structure with a periodicity of 22.7 nm, and the fundamental field is 9.3 T. (C) Zoom-in view of the high-field part of the map, marked by the white dashed rectangle in (A); scale of blue to red is 7 to 37 e2/hour. The Brown-Zak oscillations that correspond to moiré structures of different periodicities are marked by dotted lines of different colors. Black, 15.3 nm (BF = 20.5 T); green, 14.0 nm (BF = 24.2 T); brown, 11.2 nm (BF = 38 T). All measurements are performed at T = 70 K.
The behavior at high fields is more complex, as several Brown-Zak oscillations that originate from different periodicities overlap. However, by taking the periodicities, which correspond to the most prominent peaks in Rxx at B = 0 (15.3, 14, and 11.2 nm; see Fig. 2A), we could identify most oscillations in terms of fractions of the flux quantum per the corresponding plaquette, labeled in Fig. 4C. Thus, the graphene-hBN moiré periods (14.0 and 15.3 nm) and super-moiré periods (11.2, 22.7, and 36.3 nm) each give features attributable to secondary Dirac points at well-understood values of carrier concentration. In addition, they produce clear Brown-Zak oscillations for unitary flux through moiré unit cells for the 11.2-, 14.0-, 15.3-, and 22.7-nm periods.
In addition to our previous observations, we would like to note that there are several unexplained features in Rxx (Fig. 2A) and Rxy (Fig. 2B), most pronounced at ne ≈±3.2 × 1012 and ±4.1 × 1012 cm−2. One possible explanation for these features is higher-order moiré periodicities, that is, moiré patterns between super-moiré periods. However, the probability of such multiple scattering events diminishes strongly. Likewise, there could exist features due to super-moiré patterns between further zone edges (second, third,… Brillouin zone edges of the graphene/hBN moirés) or a more exotic superlattice (SL) phenomenon.
DISCUSSION
Theoretically (28), moiré effects on graphene can be described in terms of a periodic SL potential applied to Dirac electrons produced by incommensurable lattices of two (top and bottom) hBN flakes
Here, σ3 and σ = (σ1, σ2) are Pauli matrices acting in the sublattice space of graphene’s Bloch states; j = ± identifies layers α (+) and β (−);
From this, the super-moiré periods for the individual moiré SLs (α, β) in graphene originate in two ways. One is due to the quantum mechanical interference, which appears in the second-order perturbation theory. In this case, Eq. 1 allows for the electron scattering from the combined j = ±SLs with the Bragg vectors
This expression was derived for θβ ≪ δ for both parallel (P) and antiparallel (AP) mutual orientations of the two hBN crystals (hence, approximately
Vital to this description is the understanding that the displacement field u(r, R) develops because of the competition between stacking-dependent van der Waals adhesion, of graphene and hBN, and elasticity of graphene. Previous work (19, 20) has identified that the crystals form a 2D fixed-density commensurate state when graphene and hBN are close to perfect alignment. The effect relies upon strain and, thus, u(r, R), modulating with a period matching that of the moiré pattern to minimize adhesive and elastic energy. The commensurate state is characterized by hexagonal domains with increasingly sharp domain walls near θ = 0, observed in PeakForce AFM (32), and broadening of the 2D peak in the Raman spectrum (19, 25).
To evaluate the degree of strain within our super-moiré samples, we have used Raman spectroscopy. In Fig. 5A, we show the 2D peak and its full width at half maximum (FWHM) for a typical unaligned sample (black circles), a sample aligned to one hBN (blue triangles), and one of our double-aligned samples (red squares). In the case that the two hBN layers may be treated entirely independently, the signature in the Raman spectrum would remain unchanged from the single-aligned case [FWHM (2D), ~36 cm−1]. However, it is clear that when a second aligned hBN layer is added, the width of the 2D peak increases by a factor of ~2 (Fig. 5A). We attribute this to restructuring of strain within the super-moiré unit cells. This observation supports the proposed model of the two moiré patterns mixing through strain fields. Therefore, u(r, R) should have periodicities described by
Raman spectra (2D peak region) for an unaligned sample (gray), single-aligned sample (blue), and double-aligned sample (red). (A) Experimental results. (B) Molecular dynamics relaxation simulations. a.u., arbitrary units.
CONCLUSION
To conclude, graphene’s electronic spectrum is substantially altered by scattering from super-moiré structures described by the preexisting moiré between graphene and its substrate and encapsulating hBN layers. These alterations may be considered in two ways: as double-scattering events from both graphene-hBN moiré patterns or as single-scattering events from a reconstructed graphene layer. Such super-moiré potential can be of an arbitrarily small wave vector (unlike moiré potential from single hBN aligned with graphene), which allows modification of the graphene band structure at arbitrarily low energies.
MATERIALS AND METHODS
Fabrication
In addition to the sample preparation described previously, the general process is as follows. We used PPC, or sometimes polymethyl methacrylate, spun on a thick PDMS membrane to facilitate moving and orientating the crystals. This membrane was used to pick up the first hBN layer. The crystal was then positioned and aligned to graphene before the two were brought into contact. We removed the membrane quickly which lifted the graphene off from its substrate. At this point, we inverted the membrane and perform various characterization techniques on the half-assembled heterostructure. We then picked up a further thin hBN crystal, before repeating our characterization. These previous steps could have been repeated multiple times to produce increasingly complex heterostructures with a variety of crystals (although, here, we limited ourselves to graphene and hBN). Last, the stack of crystals was positioned and brought into contact with a final “substrate” hBN. The membrane was removed slowly so that all the crystals were left on a SiO2 wafer.
Atomic force microscopy
SL-resolution AFM images of the moiré patterns were taken in PeakForce Quantitative Nanomechanical Mapping (PF-QNM) mode on a Bruker Icon AFM. PF-QNM allows the capture of individual tip-sample force curves for each pixel in the image. These force curves were used to extract additional elastic information about the sample as well as topographic information. We used “ScanAsyst Fluid+” probes whose resolution is regularly better than 2 nm.
Raman spectroscopy
Raman spectroscopy measurements were performed on a HORIBA XploRA PLUS Raman spectrometer. The laser wavelength was 532 nm with a power of 0.5 mW through a ×100 objective.
Moiré Fourier analysis
The analysis of moiré patterns in double-aligned heterostructures was performed following an approach described in (33). The methodology was based on the Fourier analysis of moiré patterns appearing when two hexagonal lattices are combined.
Molecular dynamics simulations
Molecular dynamics simulations were performed for the single-aligned hBN/graphene and the double-aligned hBN/graphene/hBN by allowing the relaxations of both hBN and graphene layers. We used the bond-order Brenner potentials for the graphene layer, Tersoff potentials for the B-N interaction in the hBN layers, and the Morse potential developed in (34) for the interlayer interactions. The simulations are performed within the “large-scale atomic/molecular massively parallel simulator” (35, 36) by considering a disk of radius 120 nm. We fixed the atoms in a boundary region of 2 nm but allowed the relaxation of all other atoms. The total energy was minimized until the forces are below 10−6 eV/Å.
SUPPLEMENTARY MATERIALS
Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/12/eaay8897/DC1
More examples of double alignment
Different fundamental frequencies of Brown-Zak oscillations
Gap opening at the main Dirac point
AFM of other double-aligned samples
Uniformity in heterostructures
Analysis of super-moiré peaks
Tight-binding model
Molecular dynamics simulations and Raman shift calculations
Table S1. δ and θβ for each device.
Fig. S1. Transport properties of double-aligned encapsulated graphene devices.
Fig. S2. Brown-Zak oscillations in sample 1.
Fig. S3. Brown-Zak oscillations for sample 4.
Fig. S4. Electron-hole symmetry in super-moiré features.
Fig. S5. Gap opening in one of our double-aligned samples.
Fig. S6. Examples of double-aligned samples.
Fig. S7. Uniformity in double-aligned heterostructures.
Fig. S8. Frequency analysis of different harmonics of hBN-graphene-hBN structure.
Fig. S9. Super-moiré periods corresponding to different harmonics.
Fig. S10. Molecular dynamics simulations of bond lengths in graphene-hBN superlattices.
Reference (40)
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REFERENCES AND NOTES
- Copyright © 2019 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution License 4.0 (CC BY).
