RESUMEN
Advancements in high-temperature thermoelectric (TE) materials have been substantial, yet identifying promising near-room-temperature candidates for efficient power generation from low-grade waste heat or TE cooling applications has become critical but proven exceedingly challenging. Bismuth oxyselenide (Bi2O2Se) emerges as an ideal candidate for near-room-temperature energy harvesting due to its low thermal conductivity, high carrier mobility and remarkable air-stability. In this study, the TE properties of few-layer Bi2O2Se over a wide temperature range (20-380 K) are investigated, where a charge transport mechanism transitioning from polar optical phonon to piezoelectric scattering at 140 K is observed. Moreover, the Seebeck coefficient (S) increases with temperature up to 280 K then stabilizes atâ¼-200µV K-1through 380 K. Bi2O2Se demonstrates high mobility (450 cm2V-1s-1) within the optimum power factor (PF) window, despite itsT-1.25dependence. The high mobility compensates the minor reduction in carrier densityn2Dhence contributes to maintain a robust electrical conductivityâ¼3 × 104S m-1. This results in a remarkable PF of 860µW m-1K-2at 280 K without the necessity for gating (Vg= 0 V), reflecting the innate performance of the as-grown material. These results underscore the considerable promise of Bi2O2Se for room temperature TE applications.
RESUMEN
The authors unanimously wish to retract this Article due to their concerns about the interpretation of the low-energy electron microscopy (LEEM) and diffraction (LEED) patterns reported in the manuscript. In this study, the authors used spatial and angle-resolved photoemission spectroscopy (ARPES) to characterize graphene monolayers grown on copper foils, and observed regions of graphene adlayers with enhanced graphene/Cu interaction, higher Dirac cone doping level, moiré mini Dirac cones and large lattice expansion. All these properties have been clearly verified and reproduced by photoemission spectroscopy as well as explained by density functional theory. LEEM and LEED characterization were also carried out to confirm the existence of a moiré superlattice and lattice expansion, and the results were included in the main manuscript and Supplementary Information. On further analysis of the LEEM/LEED data, it seems that while the existence of a moiré superlattice can be corroborated, the conclusion of graphene lattice expansion (7%) based on spatially resolved ARPES determinations cannot be confirmed by the LEEM/LEED measurements. The authors realized that these measurements were collected from statistically non-representative areas of the sample. Moreover, the fact that the raw microLEED images bear an asymmetry factor of as much as 5% due to the instrumental aberration makes it impossible to estimate any compression or expansion of the same order. Consequently, their conclusion on the graphene lattice expansion can only be supported by the photoemission data. In view that more complete and reliable structural determinations should be conducted, all authors wish to retract this Article.
RESUMEN
Variations of the lattice parameter can significantly change the properties of a material, and, in particular, its electronic behaviour. In the case of graphene, however, variations of the lattice constant with respect to graphite have been limited to less than 2.5% due to its well-established high in-plane stiffness. Here, through systematic electronic and lattice structure studies, we report regions where the lattice constant of graphene monolayers grown on copper by chemical vapour deposition increases up to ~7.5% of its relaxed value. Density functional theory calculations confirm that this expanded phase is energetically metastable and driven by the enhanced interaction between the substrate and the graphene adlayer. We also prove that this phase possesses distinctive chemical and electronic properties. The inherent phase complexity of graphene grown on copper foils revealed in this study may inspire the investigation of possible metastable phases in other seemingly simple heterostructure systems.
RESUMEN
Large-area graphene film is of great interest for a wide spectrum of electronic applications, such as field effect devices, displays, and solar cells, among many others. Here, we fabricated heterostructures composed of graphene (Gr) grown by chemical vapor deposition (CVD) on copper substrate and transferred to SiO2/Si substrates, capped by n or p-type doped amorphous silicon (a-Si:H) deposited by plasma-enhanced chemical vapor deposition. Using Raman scattering we show that despite the mechanical strain induced by the a-Si:H deposition, the structural integrity of the graphene is preserved. Moreover, Hall effect measurements directly on the embedded graphene show that the electronic properties of CVD graphene can be modulated according to the doping type of the a-Si:H as well as its phase i.e. amorphous or nanocrystalline. The sheet resistance varies from 360 Ω sq(-1) to 1260 Ω sq(-1) for the (p)-a-Si:H/Gr (n)-a-Si:H/Gr, respectively. We observed a temperature independent hole mobility of up to 1400 cm(2) V(-1) s(-1) indicating that charge impurity is the principal mechanism limiting the transport in this heterostructure. We have demonstrated that embedding CVD graphene under a-Si:H is a viable route for large scale graphene based solar cells or display applications.
RESUMEN
Stacking various two-dimensional atomic crystals is a feasible approach to creating unique multilayered van der Waals heterostructures with tailored properties. Herein for the first time, we present a controlled preparation of large-area h-BN/graphene heterostructures via a simple chemical deposition of h-BN layers on epitaxial graphene/SiC(0001). Van der Waals forces, which are responsible for the cohesion of the multilayer system, give rise to an abrupt interface without interdiffusion between graphene and h-BN, as shown by X-ray Photoemission Spectroscopy (XPS) and direct observation using scanning and High-Resolution Transmission Electron Microscopy (STEM/HRTEM). The electronic properties of graphene, such as the Dirac cone, remain intact and no significant charge transfer i.e. doping, is observed. These results are supported by Density Functional Theory (DFT) calculations. We demonstrate that the h-BN capped graphene allows the fabrication of vdW heterostructures without altering the electronic properties of graphene.
RESUMEN
Chemical doping of graphene is a key process for the modulation of its electronic properties and the design and fabrication of graphene-based nanoelectronic devices. Here, we study the adsorption of diluted concentrations of nitric acid (HNO3) onto monolayer graphene/4H-SiC (0001) to induce a variation of the graphene work function (WF). Raman spectroscopy indicates an increase in the defect density subsequent to the doping. Moreover, ultraviolet photoemission spectroscopy (UPS) was utilized to quantify the WF shift. UPS data show that the WF of the graphene layer decreased from 4.3 eV (pristine) down to 3.8 eV (30% HNO3) and then increased to 4.4 eV at 100% HNO3 concentration. These observations were confirmed using density functional theory (DFT) calculations. This straightforward process allows a large WF modulation, rendering the molecularly modified graphene/4H-SiC(0001) a highly suitable electron or hole injection electrode.
RESUMEN
The conduction channel of a graphene field-effect transistor (FET) is decoupled from the parasitic charge impurities of the underlying substrate. Fluorographene as a passivation layer is fabricated between the oxide substrate and channel, and a self-aligned gate-terminated FET is also fabricated. This approach significantly reduces the scattering and, as a result, the mobility increases ten fold.