Using in situ variable-temperature scanning tunneling microscopy (300− 673 K) during chemical vapor deposition of two-dimensional hexagonal boron nitride (hBN) on Pd(111) from borazine precursor at pressures up to 10 −6 mbar, we identify the mechanisms leading to carpetlike uphill or downhill growth across the Pd steps. Deposition at a higher rate and lower temperature promotes uphill growth via preferential attachment at the ascending and descending step-edges, whereas a lower deposition rate and higher temperature lead to downhill growth via nucleation and growth of islands on Pd terraces. We attribute this unusual growth behavior to differences in temperaturedependent rates of hBN deposition at the steps versus on the Pd terraces. Our results illustrate how growth mechanisms can be activated by a pair of parameters (substrate temperature and partial pressure of borazine) and provide new insights into the mechanisms underlying carpetlike growth of hBN and other layered materials.

We show that van der Waals (vdW)-bonded hexagonal boron nitride (hBN) promotes heteroepitaxial growth of semiconducting MoS x (0001) (x = 2.0 ± 0.1) thin films on Al 2 O 3 (0001) substrates. hBN layers are grown on Al 2 O 3 (0001) via pyrolytic cracking of borazine (∼6 × 10 4 L) at 1373 K and the MoS x layers are deposited in an ultrahigh-vacuum system via reactive directcurrent magnetron sputtering of Mo in an Ar/H 2 S gas mixture at 1073 K on bare and hBN-covered Al 2 O 3 (0001). Using in situ low-energy electron diffraction and Auger electron spectroscopy along with ex situ X-ray diffraction, X-ray photoelectron and Raman spectroscopies, and transmission electron microscopy, we determine the as-deposited MoS x layer composition and crystallinity. We obtain highly 0001-oriented, ∼20-nm-thick, 2H-structured MoS x multilayers with better crystalline quality on hBN/Al 2 O 3 (0001) than on Al 2 O 3 (0001). We suggest that hBN buffer layer enhances surface diffusion of depositing species, compared to bare Al 2 O 3 (0001), leading to an observed improvement in the crystallinity of MoS x layers. We expect that our results are likely to have broad implications in nanoelectronic device fabrication.

Using a combination of in situ ultrahigh-vacuum variable-temperature scanning tunneling microscopy, ex situ Raman spectroscopy, and scanning electron microscopy, we investigated the growth of graphene using benzene on Pd(111) at temperatures up to 1100 K. Benzene adsorbs readily on Pd(111) at room temperature and forms an ordered superstructure upon annealing at 473 K. Exposure to benzene at 673 K enhances Pd step motion and yields primarily amorphous carbon upon cooling to room temperature. Monolayer graphene domains, 10–30 nm in size, appear during annealing this sample at 873 K. Dosing benzene at 1100 K results in graphene domains with varying degrees of crystallinity, while post-deposition annealing at 1100 K for 1200 s yields monolayer graphene domains larger than 150 × 150 nm2. Our results, which indicate that graphene growth on Pd(111) using benzene requires deposition/annealing temperatures higher than 673 K, are in striking contrast with the reported growth of graphene using benzene at temperatures as low as 373 K on relatively inert Cu surfaces.

Using ultra-high vacuum scanning tunneling microscopy (STM) and density functional theory (DFT), we investigated the surface structure of 2D hexagonal boron nitride (hBN) domains on Pd(1 1 1). STM images of polydomain hBN monolayers, grown via dissociative chemisorption of borazine on Pd(1 1 1)/Al2O3(0 0 0 1) thin films, are acquired as a function of tunneling current and bias. The images reveal moiré patterns with four periodicities λ  =  0.6  ±  0.05 nm, 1.8  ±  0.14 nm, 2.7  ±  0.20 nm, and 2.8  ±  0.14 nm, corresponding to different orientations on Pd(1 1 1). We find that the apparent surface corrugation Δz in STM changes little with tunneling current, exhibits an oscillatory dependence on the bias voltage, and increases from Δz  ≈  14 pm for domains with λ  =  0.6 nm to Δz  ≈  200 pm for λ  =  2.8 nm. We attribute the observed tunneling-parameter dependence in Δz to the electronic structure of the hBN/Pd(1 1 1) system. Unlike any other monolayer hBN-on-metal system, we suggest that hBN/Pd can have either mainly geometric or mainly electronic corrugation, depending on the domain orientation. Furthermore, for the largest periodicities, we observe a bifurcation behavior in which some domains are nearly flat, and others develop significant hill-and-valley geometric corrugations. We expect a similar behavior for other substrates for which the interaction energy with hBN is intermediate, i.e. neither mostly chemical nor van der Waals binding: for these substrates, a similar approach can help identify interlayer interactions and electronic structure modifications.

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