Controlling the stacking order in bilayer graphene (BLG) allows realizing interesting physical properties. In particular, the possibility of tuning the band gap in Bernal-stacked (AB) BLG (AB-BLG) has a great technological importance for electronic and optoelectronic applications. Most of the current methods to produce AB-BLG suffer from inhomogeneous layer thickness and/or coexistence with twisted BLG. Here, we demonstrate a method to synthesize highly pure large-area AB-BLG by chemical vapor deposition using Cu–Ni films. Increasing the reaction time resulted in a gradual increase of the AB stacking, with the BLG eventually free from twist regions for the longer growth times (99.4% of BLG has AB stacking), due to catalyst-assisted continuous BLG reconstruction driven by carbon dissolution–segregation processes. The band gap opening was confirmed by the electrical measurements on field-effect transistors using two different device configurations. The concept of the continuous reconstruction to achieve highly pure AB-BLG offers a way to control the stacking order of catalytically grown two-dimensional materials.

2D PdSe2, a Group 10 noble metal dichalcogenide, has been reported to have a strong thickness‐dependent bandgap energy, ranging from ≈1.6 eV (monolayer) to ≈0.05 eV (bulk) and a high photoresponsivity for bulk samples in the far infrared wavelength range of 10.6 μm. However, a middle bandgap of ≈0.5 eV has been contradictorily reported for bulk PdSe2 via optical absorption measurements. In this study, detailed electrical transport measurements are conducted to solve this contradiction. The key difference between narrow gap and middle gap semiconductors is the contribution of a depletion layer to the transfer characteristics. Hall measurements reveal intrinsic p‐type carrier densities of ≈1.9 × 1018 cm−3 at 300 K and the contribution of the depletion layer to the transfer characteristics for bulk PdSe2, suggesting a middle bandgap. Moreover, the maximum depletion width (WDm) is determined from top‐ and back‐gate coupling in dual gate transistors to be ≈17–18 nm. Based on the WDm – acceptor density diagram, the bandgap of bulk PdSe2 is quantitatively estimated to be ≈0.3 eV. Although this is not a desirable result from the viewpoint of far infrared material, it helps us to correctly understand the mechanism for the photoresponse of PdSe2.

of InN with high mobility was initially estimated to be 1.89 eV from the optical absorption of polycrystalline films. [1,2] Since the large amounts of defects resulted in the degenerate doping of carriers, E G determined by optical absorption apparently increased, [2] which is called as the Moss-Burstein effect. [3] In fact, highquality single-crystal InN indicated that E G was actually as small as 0.7 eV. [4] Currently, this small bandgap plays an important role in the alloying design for indium gallium nitride semiconductors, and the developments of solar cells are being actively conducted.Here, in the area of 2D materials for which many new materials are being synthesized, [5][6][7] PdSe 2 is the focus because its bandgap has been reported to be located in the far-infrared (FIR) region. It is one of the group 10 noble metal dichalcogenides (NMDCs), and there are a few reports in the 1960s. [8] The E G for bulk PdSe 2 was reported to be %0.4 eV from the temperature dependence of resistivity ρ ∝ expðE G =2k B TÞ under the assumption of an intrinsic semiconductor, where k B and T are the Boltzmann constant and temperature, respectively. [9] This semiconducting nature is systematically understood by the following that electronic structures depend on progressive filling of the d bands and that the Fermi level for PdSe 2 with d 6 is located between the topmost d band and the antibonding (σ*) band. [8,10,11] After the stagnated period, PdSe 2 crystals were resynthesized by a self-flux method in 2017, [12] inspired by density functional theory (DFT) calculations on unique properties derived from a puckered pentagonal structure and a widely tunable E G from 1.43 eV for a monolayer to 0.03 eV for bulk. [13][14][15] The absorption measurement of exfoliated flakes showed negligible E G for bulk and %1.3 eV for a monolayer [12] ; these values are basically consistent with those predicted by DFT calculations. Then, in 2019, a highly sensitive bulk PdSe 2 photodetector was demonstrated at an FIR wavelength of 10.6 μm, and the mechanism was explained by the photogating effect, in which electron-hole pairs are generated through the bandgap and holes trapped at interface traps act as localized floating gates. [16] Therefore, PdSe 2 has attracted much attention as an optoelectronic material, [17][18][19][20][21][22][23][24] focusing on infrared detectors, as shown in Figure S1, Supporting Information. Since HgCdTe [25,26] and graphene are the only materials that have a bandgap in the FIR region thus far,