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1.
Nat Commun ; 10(1): 5013, 2019 Nov 01.
Artigo em Inglês | MEDLINE | ID: mdl-31676774

RESUMO

For atomically thin two-dimensional materials, interfacial effects may dominate the entire response of devices, because most of the atoms are in the interface/surface. Graphene/sapphire has great application in electronic devices and semiconductor thin-film growth, but the nature of this interface is largely unknown. Here we find that the sapphire surface has a strong interaction with some of the carbon atoms in graphene to form a C-O-Al configuration, indicating that the interface interaction is no longer a simple van der Waals interaction. In addition, the structural relaxation of sapphire near the interface is significantly suppressed and very different from that of a bare sapphire surface. Such an interfacial C-O-Al bond is formed during graphene growth at high temperature. Our study provides valuable insights into understanding the electronic structures of graphene on sapphire and remote control of epitaxy growth of thin films by using a graphene-sapphire substrate.

2.
Nano Lett ; 19(10): 7124-7129, 2019 Oct 09.
Artigo em Inglês | MEDLINE | ID: mdl-31545615

RESUMO

We study the impact of organic surface ligands on the electronic structure and electronic band edge energies of quasi-two-dimensional (2D) colloidal cadmium selenide nanoplatelets (NPLs) using density functional theory. We show how control of the ligand and ligand-NPL interface dipoles results in large band edge energy shifts, over a range of 5 eV for common organic ligands with a minor effect on the NPL band gaps. Using a model self-energy to account for the dielectric contrast and an effective mass model of the excitons, we show that the band edge tunability of NPLs together with the strong dependence of the optical band gap on NPL thickness can lead to favorable photochemical and optoelectronic properties.

3.
Adv Mater ; 31(23): e1807345, 2019 Jun.
Artigo em Inglês | MEDLINE | ID: mdl-30993771

RESUMO

The growth of single-crystal III-nitride films with a low stress and dislocation density is crucial for the semiconductor industry. In particular, AlN-derived deep-ultraviolet light-emitting diodes (DUV-LEDs) have important applications in microelectronic technologies and environmental sciences but are still limited by large lattice and thermal mismatches between the epilayer and substrate. Here, the quasi-van der Waals epitaxial (QvdWE) growth of high-quality AlN films on graphene/sapphire substrates is reported and their application in high-performance DUV-LEDs is demonstrated. Guided by density functional theory calculations, it is found that pyrrolic nitrogen in graphene introduced by a plasma treatment greatly facilitates the AlN nucleation and enables fast growth of a mirror-smooth single-crystal film in a very short time of ≈0.5 h (≈50% decrease compared with the conventional process), thus leading to a largely reduced cost. Additionally, graphene effectively releases the biaxial stress (0.11 GPa) and reduces the dislocation density in the epilayer. The as-fabricated DUV-LED shows a low turn-on voltage, good reliability, and high output power. This study may provide a revolutionary technology for the epitaxial growth of AlN films and provide opportunities for scalable applications of graphene films.

4.
Adv Mater ; 30(30): e1801608, 2018 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-29883036

RESUMO

Single-crystalline GaN-based light-emitting diodes (LEDs) with high efficiency and long lifetime are the most promising solid-state lighting source compared with conventional incandescent and fluorescent lamps. However, the lattice and thermal mismatch between GaN and sapphire substrate always induces high stress and high density of dislocations and thus degrades the performance of LEDs. Here, the growth of high-quality GaN with low stress and a low density of dislocations on graphene (Gr) buffered sapphire substrate is reported for high-brightness blue LEDs. Gr films are directly grown on sapphire substrate to avoid the tedious transfer process and GaN is grown by metal-organic chemical vapor deposition (MOCVD). The introduced Gr buffer layer greatly releases biaxial stress and reduces the density of dislocations in GaN film and Inx Ga1-x N/GaN multiple quantum well structures. The as-fabricated LED devices therefore deliver much higher light output power compared to that on a bare sapphire substrate, which even outperforms the mature process derived counterpart. The GaN growth on Gr buffered sapphire only requires one-step growth, which largely shortens the MOCVD growth time. This facile strategy may pave a new way for applications of Gr films and bring several disruptive technologies for epitaxial growth of GaN film and its applications in high-brightness LEDs.

5.
Phys Chem Chem Phys ; 20(8): 5699-5707, 2018 Feb 21.
Artigo em Inglês | MEDLINE | ID: mdl-29410993

RESUMO

The effects of the staggered double vacancies, hydrogen (H), 3d transition metals, for example cobalt, and semiconductor covalent atoms, for example, germanium, nitrogen, phosphorus (P) and silicon adsorption on the transport properties of monolayer phosphorene were studied using density functional theory and non-equilibrium Green's function formalism. It was observed that the performance of the phosphorene tunnel field effect transistors (TFETs) with an 8.8 nm scaling channel length could be improved most effectively, if the adatoms or vacancies were introduced at the source channel interface. For H and P doped devices, the upper limit of on-state currents of phosphorene TFETs were able to be quickly increased to 2465 µA µm-1 and 1652 µA µm-1, respectively, which not only outperformed the pristine sample, but also met the requirements for high performance logic applications for the next decade in the International Technology Roadmap for Semiconductors (ITRS). It was proved that the defect-induced band gap states make the effective tunneling path between the conduction band (CB) and valence band (VB) much shorter, so that the carriers can be injected easily from the left electrode, then transfer to the channel. In this regard, the tunneling properties of phosphorene TFETs can be manipulated using surface defects. In addition, the effects of spin polarization on the transport properties of doped phosphorene TFETs were also rigorously considered, H and P doped TFETs could achieve a high ON current of 1795 µA µm-1 and 1368 µA µm-1, respectively, which is closer to realistic nanodevices.

6.
Nano Lett ; 12(1): 383-8, 2012 Jan 11.
Artigo em Inglês | MEDLINE | ID: mdl-22192078

RESUMO

Semiconductor photocatalysts capable of broadband solar photon absorption may be nonetheless precluded from use in driving water splitting and other solar-to-fuel related reactions due to unfavorable band edge energy alignment. Using first-principles density functional theory and beyond, we calculate the electronic structure of passivated CdSe surfaces and explore the opportunity to tune band edge energies of this and related semiconductors via electrostatic dipoles associated with chemisorbed ligands. We predict substantial shifts in band edge energies originating from both the induced dipole at the ligand/CdSe interface and the intrinsic dipole of the ligand. Building on important induced dipole contributions, we further show that, by changing the size and orientation of the ligand's intrinsic dipole moment via functionalization, we can control the direction and magnitude of the shifts of CdSe electronic levels. Our calculations suggest a general strategy for enabling new active semiconductor photocatalysts with both optimal opto-electronic, and photo- and electrochemical properties.


Assuntos
Compostos de Cádmio/química , Compostos de Cádmio/efeitos da radiação , Modelos Químicos , Nanoestruturas/química , Nanoestruturas/efeitos da radiação , Compostos de Selênio/química , Compostos de Selênio/efeitos da radiação , Catálise/efeitos da radiação , Simulação por Computador , Transferência de Energia , Ligantes , Luz , Fótons
7.
Nano Lett ; 10(8): 3156-62, 2010 Aug 11.
Artigo em Inglês | MEDLINE | ID: mdl-20698631

RESUMO

Using first-principles calculations within density functional theory, we study the relative impacts of quantum confinement and strain on the electronic structure of two II-VI semiconductor compounds with a large lattice-mismatch, CdSe and CdTe, in core/shell nanowire geometries with different core radii and shell thicknesses. For fixed CdSe core radius, we find that the electronic band gap in the core is significantly reduced with increasing CdTe shell thickness, by an amount comparable to that expected from quantum confinement, due to the development of a large and highly anisotropic strain throughout the heterostructure. A straightforward analysis allows us to separate quantitatively changes in band gap due to quantum confinement and strain. Our studies elucidate and quantify the importance of strain in determining the electronic and optical properties of core/shell nanostructures.

8.
J Chem Phys ; 131(6): 064707, 2009 Aug 14.
Artigo em Inglês | MEDLINE | ID: mdl-19691403

RESUMO

Within first-principles density functional theory, we explore the feasibility of using metallofullerenes as efficient hydrogen storage media. In particular, we systematically investigate the interaction between hydrogen molecules and La encapsulated all-carbon fullerenes, C(n) (20 < or = n < or = 82), focusing on the role of transferred charges between the metal atoms and fullerenes on the affinity of hydrogen molecules to the metallofullerenes. Our calculations show that three electrons are transferred from La atom to fullerene cages, while the induced charges are mostly screened by the fullerene cages. We find the local enhancement of molecular hydrogen affinity to the fullerenes to be sensitively dependent on the local bonding properties, rather than on the global charging effects.

9.
J Chem Phys ; 129(13): 134707, 2008 Oct 07.
Artigo em Inglês | MEDLINE | ID: mdl-19045116

RESUMO

Using ab initio spin density functional theory, we investigate the energetics and kinetics of Ti clustering on both neutral and charged C(60) surfaces. We compare the formation energy of sparsely dispersed zero-dimensional (0D), compact single-layered two-dimensional (2D), and clustered three-dimensional (3D) Ti(N) configurations as a function of cluster size (N < or = 12) and further study the transformation kinetics between them. We find that 0D configuration is always less stable than that of 2D and 3D configurations and 0D to 2D transformation involves in a single Ti diffusion process with kinetic barrier of < or = 0.7 eV. On the other hand, there exists a critical cluster size (N(C)) of N(C) = 5, below which 2D layers are preferred to 3D clusters. Hole- or B-doping greatly enhance the Ti-fullerene interaction and lead to stronger dispersion of Ti atoms. Even so, for moderate charge doping (less than seven holes) the critical size of Ti atoms on neutral C(60) surprisingly remains unchanged or only slightly increases to N(C) = 6 by B-doping. However, we find that the formation of 3D clusters may be hindered by a high kinetic barrier related to the process of single Ti atoms climbing up a single Ti layer. This barrier is approximately 1 eV or even 1.47 eV for B-doped C(60) surfaces which is high enough to stabilize larger 2D structures (N > or = N(C)) at low temperatures. These findings may prove to be instrumental in stabilizing transition metal coated nanostructures and especially homogeneously Ti-coated fullerenes, which are believed to be a very promising material for hydrogen storage.

10.
Phys Rev Lett ; 100(20): 206806, 2008 May 23.
Artigo em Inglês | MEDLINE | ID: mdl-18518569

RESUMO

We explore theoretically the feasibility of functionalizing carbon nanostructures for hydrogen storage, focusing on the coating of C60 fullerenes with light alkaline-earth metals. Our first-principles density functional theory studies show that both Ca and Sr can bind strongly to the C60 surface, and highly prefer monolayer coating, thereby explaining existing experimental observations. The strong binding is attributed to an intriguing charge transfer mechanism involving the empty d levels of the metal elements. The charge redistribution, in turn, gives rise to electric fields surrounding the coated fullerenes, which can now function as ideal molecular hydrogen attractors. With a hydrogen uptake of >8.4 wt % on Ca32C60, Ca is superior to all the recently suggested metal coating elements.

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