Solution-processed hybrid organic–inorganic lead halide perovskites are emerging as one of the most promising candidates for low-cost light-emitting diodes (LEDs). However, due to a small exciton binding energy, it is not yet possible to achieve an efficient electroluminescence within the blue wavelength region at room temperature, as is necessary for full-spectrum light sources. Here, we demonstrate efficient blue LEDs based on the colloidal, quantum-confined 2D perovskites, with precisely controlled stacking down to one-unit-cell thickness (n = 1). A variety of low-k organic host compounds are used to disperse the 2D perovskites, effectively creating a matrix of the dielectric quantum wells, which significantly boosts the exciton binding energy by the dielectric confinement effect. Through the Förster resonance energy transfer, the excitons down-convert and recombine radiatively in the 2D perovskites. We report room-temperature pure green (n = 7–10), sky blue (n = 5), pure blue (n = 3), and deep blue (n = 1) electroluminescence, with record-high external quantum efficiencies in the green-to-blue wavelength region.
Gate-tunable two-dimensional (2D) materials-based quantum capacitors (QCs) and van der Waals heterostructures involve tuning transport or optoelectronic characteristics by the field effect. Recent studies have attributed the observed gate-tunable characteristics to the change of the Fermi level in the first 2D layer adjacent to the dielectrics, whereas the penetration of the field effect through the one-molecule-thick material is often ignored or oversimplified. Here, we present a multiscale theoretical approach that combines first-principles electronic structure calculations and the Poisson–Boltzmann equation methods to model penetration of the field effect through graphene in a metal–oxide–graphene–semiconductor (MOGS) QC, including quantifying the degree of “transparency” for graphene two-dimensional electron gas (2DEG) to an electric displacement field. We find that the space charge density in the semiconductor layer can be modulated by gating in a nonlinear manner, forming an accumulation or inversion layer at the semiconductor/graphene interface. The degree of transparency is determined by the combined effect of graphene quantum capacitance and the semiconductor capacitance, which allows us to predict the ranking for a variety of monolayer 2D materials according to their transparency to an electric displacement field as follows: graphene > silicene > germanene > WS2 > WTe2 > WSe2 > MoS2 > phosphorene > MoSe2 > MoTe2, when the majority carrier is electron. Our findings reveal a general picture of operation modes and design rules for the 2D-materials-based QCs.
Dielectric Screening in Atomically Thin Boron Nitride Nanosheetshttp://dx.doi.org/10.1021/acs.nanolett.6b01876, Nano Letters, 2015, 15, No. 1, pp. 218 doi: 10.1021/nl503411aAbstract
Two-dimensional (2D) hexagonal boron nitride (BN) nanosheets are excellent dielectric substrate for graphene, molybdenum disulfide, and many other 2D nanomaterial-based electronic and photonic devices. To optimize the performance of these 2D devices, it is essential to understand the dielectric screening properties of BN nanosheets as a function of the thickness. Here, electric force microscopy along with theoretical calculations based on both state-of-the-art first-principles calculations with van der Waals interactions under consideration, and nonlinear Thomas–Fermi theory models are used to investigate the dielectric screening in high-quality BN nanosheets of different thicknesses. It is found that atomically thin BN nanosheets are less effective in electric field screening, but the screening capability of BN shows a relatively weak dependence on the layer thickness.
Dr. Santos was educated at the Danish Technical University and the University of the Basque Country where he received a European M.Sc. (2008) and Ph.D. (2011) in Nanoscience and Physics, respectively.
He was awarded the highly prestigious and competitive SEAS Fellowship from the Harvard School of Engineering and Applied Sciences at Harvard University to perform postdoctoral research in computational materials science in energy and functional materials. While in Harvard SEAS he was awarded three different prizes from the American Physical Society (APS), which include the 2012, and 2013 APS March Meeting Image Gallery Awards, and the 2013 APS March Meeting Cover Award. He moved to Stanford University in 2013 to be named SunCat Research Fellow at the Department of Chemical Engineering and SLAC National Laboratory.
He has recently been pointed Queen's Fellow at Queen's University Belfast, with a joint Faculty Position (Assistant Professorship) at the School of Mathematics and Physics and School of Chemistry and Chemical Engineering.
Dr. Santos' research interests span a range of topics in the physics and chemistry of solids and molecules and the use and development of first-principles simulations to address problems such as: the electronic and optical properties of crystalline materials and their dependence on the atomic structure, chemical composition and applied external driving forces (e.g. strain, electric fields, temperature, etc.); photo-chemistry and light-driven chemical reactions; the nature of electronic states, dielectric properties and epitaxial relationships of 2D-nanomaterials, in particular studying the interactions between organic crystals and heterostructures; the microscopic control of graphene nanopores for DNA sequencing. Recent applications of the computational models that he developed focus on discovering new materials and processes for solar energy conversion and energy fuel storage, for instance in photocatalysis and photovoltaics. He has been recently awarded the Interfolio Scholar Gallery Prize from Interfolio Inc., and he is part of the editorial board of Advanced Materials Science and Applications and Graphene journals.
List of my publications (Google Scholar)