Surface termination and strain-induced modulation of the structure and electronic properties in 2D perovskites (Cs2BCl4 & CsB2Cl5, B = Pb, Sn): a first-principles study.

Two-dimensional (2D) halide perovskites have demonstrated impressive long-term stability and superior device performance as compared to their three-dimensional (3D) counterparts. The potential of 2D halide perovskites for advanced photovoltaic applications can be enhanced by an understanding of how external factors like strain could be used to tune their optoelectronic properties. This study explores the effects of biaxial strain on the structure and electronic transport properties of 2D halide perovskites, focusing on the lowest energy (001) surfaces of (Cs2BCl4 and CsB2Cl5, B = Pb or Sn) with CsCl and BCl2 terminations. Using first-principles calculations, we find that the lower energy CsCl terminated surface, resulting in Cs2BCl4, couples strongly with biaxial strain. This termination shows bandgap modulations from approximately 1.5 eV to 1.8 eV for Cs2PbCl4 and 1.2 eV to 1.5 eV for Cs2SnCl4 with biaxial strain. Within the acoustic deformation potential theory, we compute hole mobilities, and find substantial enhancements of approximately 80% for Pb-based and 50% for Sn-based systems, thereby emphasizing the potential of strain engineering to further optimize charge transport properties in 2D halide perovskites.

Aluminene as a Low-Cost Anode Material for Li- and Na-Ion Batteries.

Two-dimensional (2D) materials are promising candidates for next-generation battery technologies owing to their high surface area, excellent electrical conductivity, and lower diffusion energy barriers. In this work, we use first-principles density functional theory to explore the potential for using a 2D honeycomb lattice of aluminum, referred to as aluminene, as an anode material for metal-ion batteries. The metallic monolayer shows strong adsorption for a range of metal atoms, i.e., Li, Na, K, and Ca. We observe surface diffusion barriers as low as 0.03 eV, which correlate with the size of the adatom. The relatively low average open-circuit voltages of 0.27 V for Li and 0.42 V for Na are beneficial to the overall voltage of the cell. The estimated theoretical specific capacity has been found to be 994 mA h/g for Li and 870 mA h/g for Na. Our research highlights the promise of aluminene sheets in the development of low-cost, high-capacity, and lightweight advanced rechargeable ion batteries.

Hydrogen adsorption and diffusion through a two dimensional sheet of lithium: a first principles study.

Two-dimensional (2D) materials have shown promise as highly selective, ultrathin membranes to transport ions, and atomic and subatomic particles. They have also been regarded as potential hydrogen storage candidates due to their chemical stability and high specific surface area. However, most of these studies have been carried out with semiconducting 2D materials. With recent explorations towards the existence and stability of 2D metals, we explore the hydrogen adsorption and diffusion through a 2D metallic sheet of lithium. We report that in the lowest energy metallic configuration, the sheet is predicted to crystallize in a highly buckled honeycomb structure. We calculate the adsorption energy for the diffusion of hydrogen on various high symmetry sites in the lattice, and find that adsorption is energetically favoured. We study the minimum energy pathways for diffusion through the sheet and find that the lowest energy barriers exist for tunneling through the honeycomb ring. Our results would be of direct technological relevance to the applications of 2D metallic nanostructures as membranes for selective transport or towards storage.

Carrier Induced Hopping to Band Conduction in Pentacene.

Charge transport in organic thin films which are generally polycrystalline is typically limited by the localization of the carriers at lattice defects resulting in low carrier mobilities and carriers move from one state to another state by hopping. However, charge transport in organic semiconductors in their single crystalline phase is coherent due to band conduction and mobilities are not limited by disorder resulting in higher carrier mobility. So it is a challenge to enhance the carrier mobility in a thin film which is the preferred choice for all organic devices. Here, we show that it is possible to increase the carrier mobility in polycrystalline thin films by injecting sufficient carriers such that Fermi level can be moved into the region of high density in Gaussian density of states of molecular solids. When the hopping transport happens through the molecular energy levels whose density is low, mobility is decided by incoherent transport however, when the the hopping transport happens through the energy levels with high density, mobility is decided by coherent transport, as in band conduction. We present results highlighting the observation of both band-like and hopping conduction in polycrystalline organic thin films by varying the concentration of injected charge. More importantly the transition from hopping to band transport is reversible. The observed carrier mobilities in both the regimes match well with theoretical estimates of hopping mobility and band mobility determined from first principles density functional theory.

Influence of Structure on Electronic Charge Transport in 3D Ge Nanowire Networks in an Alumina Matrix.

We demonstrate formation of material consisting of three-dimensional Germanium nanowire network embedded in an insulating alumina matrix. A wide range of such nanowire networks is produced using a simple magnetron sputtering deposition process. We are able to vary the network parameters including its geometry as well as the length and width of the nanowires. The charge transport in these materials is shown to be related to the nanowire surface per unit volume of the material, α. For low values of α, transport is characterized by space charge limited conduction and a drift of carriers in the extended states with intermittent trapping-detrapping in the localized states. For large values of α, charge transport occurs through hopping between localized electronic states, similar to observations in disorder-dominated arrays of quantum dots. A crossover between these two mechanisms is observed for the intermediate values of α. Our results are understood in terms of an almost linear scaling of the characteristic trap energy with changes in the nanowire network parameters.

Tuning the Electronic and Magnetic Properties of CuAlO2 Nanocrystals Using Magnetic Dopants.

Optoelectronic applications with transparent conducting oxides have been made possible by modulating the carrier density of wide band gap oxides with doping. We demonstrate the modulation of the density of states (DOS) at the Fermi level in nanocrystalline CuAlO2 particles synthesized using a sol-gel technique, as a function of doping with a magnetic impurity (Ni). This behavior is directly correlated with structural studies using X-ray diffraction and magnetic properties which show a similar trend. Our results can be understood in a picture where charge hopping occurs through surface or defect states, rather than by direct hopping between the quantum-confined states of the nanocrystal, and an increase in the DOS at the Fermi level caused by the substitution of Ni atoms at the Al site.

Measuring Ligand-Dependent Transport in Nanopatterned PbS Colloidal Quantum Dot Arrays Using Charge Sensing.

Colloidal quantum dot arrays with long organic ligands have better packing order than those with short ligands but are highly resistive, making low-bias conductance measurements impossible with conventional two-probe techniques. We use an integrated charge sensor to study transport in weakly coupled arrays in the low-bias regime, and we nanopattern the arrays to minimize packing disorder. We present the temperature and field dependence of the resistance for nanopatterned oleic-acid and n-butylamine-capped PbS arrays, measuring resistances as high as 10(18) Ω. We find that the conduction mechanism changes from nearest neighbor hopping in oleic-acid-capped PbS dots to Mott's variable range hopping in n-butylamine capped PbS dots. Our results can be understood in terms of a change in the interdot coupling strength or a change in density of trap states and highlight the importance of the capping ligand on charge transport through colloidal quantum dot arrays.

Nanopatterned electrically conductive films of semiconductor nanocrystals.

We present the first semiconductor nanocrystal films of nanoscale dimensions that are electrically conductive and crack-free. These films make it possible to study the electrical properties intrinsic to the nanocrystals unimpeded by defects such as cracking and clustering that typically exist in larger-scale films. We find that the electrical conductivity of the nanoscale films is 180 times higher than that of drop-cast, microscopic films made of the same type of nanocrystal. Our technique for forming the nanoscale films is based on electron-beam lithography and a lift-off process. The patterns have dimensions as small as 30 nm and are positioned on a surface with 30 nm precision. The method is flexible in the choice of nanocrystal core-shell materials and ligands. We demonstrate patterns with PbS, PbSe, and CdSe cores and Zn(0.5)Cd(0.5)Se-Zn(0.5)Cd(0.5)S core-shell nanocrystals with a variety of ligands. We achieve unprecedented versatility in integrating semiconductor nanocrystal films into device structures both for studying the intrinsic electrical properties of the nanocrystals and for nanoscale optoelectronic applications.