School of Materials and Energy

We report a significant broadband enhancement of the external quantum efficiency of the quantum dot solar cell by coupling with plasmonic nanostars via a simple and scalable "boiling deposition" technique. The multispiked nanostars provide broadband scattering and absorption cross-sections, which can be engineered to dramatically boost the performance of the solar cells. The localized near field modes of nanostars result in an external quantum efficiency enhancement over 400% for short-wavelength light absorbed in the emitter, while plasmon light scattering causes distinct improvement in quantum efficiency (10-50%) in the long-wavelength region up to 1100 nm. Finite difference time domain method is adopted to explain the origin of the optical absorption enhancement in the quantum dot solar cells. The broadband light

Strain analysis has significance both for tailoring material properties and designing nanoscale devices. In particular, strain plays a vital role in engineering the growth thermodynamics and kinetics and is applicable for designing optoelectronic devices. In this paper, we present a methodology for establishing the relationship between elastic bond constants and measurable parameters, i.e., Poisson's ratio ν and systematic elastic constant K. At the atomistic level, this approach is within the framework of linear elastic theory and encompasses the neighbor interactions when an atom is introduced to stress. Departing from the force equilibrium equations, the relationships between ν, K, and spring constants are successfully established. Both the two-dimensional (2D) square lattice and common three-dimensional (3D) structures are taken into account in the procedure for facilitating, bridging the gap between structural complexity and numerical experiments. A new direction for understanding the physical phenomena in strain engineering is established.

Massive efforts have been devoted to enhancing performances of Li−S batteries to meet the requirements of practical applications. However, problems remain in enhancing the energy density and improving the cycle life. We present a free-standing structure of walnut-shaped VS 4 nanosites combine with carbon nanotubes (NTs) as cathodes. In this framework, NT arrays provide high surface area and conductivity for high sulfur loadings, and VS 4 nanosites facilitate trapping and catalytic conversions of lithium polysulfides. The synergistic effects of free-standing NT arrays and VS 4 nanosites have enabled high rate capability up to 6 C and longterm cycling with a low decay rate of 0.037% up to 1200 cycles at 2 C. Moreover, the designed cathode can achieve high areal capacities up to ∼13 mAh•cm −2 and estimated gravimetric energy density of 243.4 Wh• kg −1 at a system level, demonstrating great potential in practical applications of Li−S batteries.

Carbon nitride covalent compounds have emerged as a prominent member of 2D materials beyond graphene. The experimental realizations of 2D graphitic carbon nitride g-C 3 N 4 , nitrogenated holey graphene C 2 N, and polyaniline C 3 N have shown their promising potential in energy and environmental applications. In this work, we predict a new type of carbon nitride network with a C 9 N 4 stoichiometry from first principles calculations. Unlike common C-N compounds and covalent organic frameworks (COFs), which are typically insulating, surprisingly C 9 N 4 is found to be a 2D nodal-line semimetal. The nodal line in C 9 N 4 forms a closed ring centered at the G point, which originates from the p z orbitals of both C and N. The linear crossing occurs right at the Fermi level contributed by two sets of dispersive Kagome and Dirac bands, which is robust due to negligible spin-orbit coupling in C and N. Furthermore, it is revealed that the degeneracy along the high-symmetry path is protected by out-of-plane mirror or C 2 rotational symmetry, rather than in-plane mirror symmetry. The chemical potential difference between C and N, as validated by using a single orbital tight-binding model, plays a significant role in forming the nodal ring. Interestingly, a new structure of the nodal line, i.e., a nodal cylinder, is found in momentum space for AA-stacked C 9 N 4. Our results indicate possible functionalization for a novel metalfree C-N covalent network with interesting semimetallic properties.

Recent developments in synthesis and nanofabrication technologies
offer the tantalizing prospect of realizing various applications from two-dimensional (2D) materials. A revolutionary development is to flexibly construct many different kinds of heterostructures with a diversity of 2D materials. These 2D heterostructures play an important role in semiconductor and condensed matter physics studies and are promising candidates for new device designs in the fields of integrated circuits and quantum sciences. Theoretical and experimental studies have focused on both vertical and lateral 2D heterostructures; the lateral heterostructures are considered to be easier for planner integration and exhibit unique electronic and photoelectronic properties. In this review, we give a summary of the properties of lateral heterostructures with homogeneous junction and heterogeneous junction, where the homogeneous junctions have the same host materials and the heterogeneous junctions are combined with different materials. Afterward, we discuss the applications and experimental synthesis of lateral 2D heterostructures. Moreover, a perspective on lateral 2D heterostructures is given at the end.

Modulating the electronic properties of the twodimensional lateral heterostructures (2DLH) comprising one single class of materials is still a challenge. The understanding of the electronic properties of band alignments is highly needed for the demands of pertinent electronic devices. Herein, steered via first-principles calculations, three different methods including antidot-shape modification, width tailoring, and doping are applied to effectively modulate the electronic properties of an armchair graphene antidot nanoribbon (AGANR). As a result, the formation of 2DLH with both type-I and type-II band alignment can be achieved. Especially, a reference table of possible formation of heterostructures is presented, which is a recipe for designing electronic devices with different requirements. In addition, the gap scaling rule with odd−even characteristics of the AGANRs through width controlling is revealed that is beneficial for the computational screening of functional materials based on the energy gap. Finally, the transport properties of two typical heterostructures show the manifestation of the type-I and type-II band alignment. Our results provide the diverse schemes to realize two types of band alignments based on AGANR, which may have potential applications in optical devices like light-emitting diodes with type-I heterostructures and carrier separation like solar cells with type-II ones, respectively.

Developing novel low-cost and efficient electron transport layer (ETL) materials for high-efficiency planar heterojunction perovskite solar cells (PSCs) still remains challenges. Herein, we report a low-temperature colloid-synthesized and solution-deposited strontium stannate SrSnO 3 (SSO) perovskite oxide nanoparticle can be an effective alternative ETL. More importantly, the introduction of yttrium dopant results in a significant improvement in the optoelectronic properties of yttrium-doped SrSnO 3 (YSSO), exhibiting higher electron conductivity and faster electron transfer, as well as better band alignment at ETL/perovskite interface compare to undoped SSO, which is also supported by theory calculation. Consequently, these factors boost all photovoltaic performance parameters, leading to an improvement in the efficiency of planar PSCs. The resulting YSSO-based PSCs exhibit an average efficiency of 17.8% and a maximum power conversion efficiency of 19.0% with the significant reduction of J-V hysteresis, and those devices show high long-term stability as well. Our findings provide the full potential of the perovskite oxide toward future photovoltaic applications, especially for costefficient planar PSCs.

Two-dimensional hexagonal boron nitride (h-BN) single crystals with various shapes have been synthesized by chemical vapor deposition over the past several years. Here, we report the formation of three-leaf dart (3LD)-shaped single crystals of h-BN on a Cu foil by atmospheric-pressure
chemical vapor deposition. The leaves of the 3LD-shaped h-BN are as long as 18 lm, and their edges are smooth armchair on one side and stepped armchair on the other. Careful analysis revealed that surface oxygen plays an important role in the formation of the 3LD shape. Oxygen suppressed h-BN nucleation by passivating Cu surface active sites and lowered the edge attachment energy, which caused the growth kinetics to change to a diffusion-controlled mode.

Potassium-ion batteries (KIBs) are potential substitutions of lithium-ion batteries, considering the superiority of K in abundance and safety compared with lithium. Recently, the MXenes emerged as a significant class of twodimensional materials in energy storage applications. Herein we investigated the performances of two S-functionalized MXenes (S-MXenes), Ti2CS 2 and Ti2NS2 , as anodes for KIBs through first-principle calculations. The results show that both S-MXenes possess metallic nature and structural integrity. Besides, the two functionalized structures exhibit strong adsorptions and low diffusion energy barriers, suggesting fast charge-discharge rates. By reaching the ultimate structural configurations, the theoretical capacities are as high as 312 mAh/g with low but positive open circuit voltage. All the performances reveal that the S-MXenes are promising anode materials for KIBs and expand their application as electrode materials.

Complete configurations of monolayer graphene with zigzag-edged hexagonal nanohole are systematically built using three indices. The three integers lead to an atomistic precisely gap opening criteria, which steered via density functional theory based calculations. The guiding rule reveals that the semimetallic and semiconducting variation is consistent with a peculiar sequence with a period of 3 for odd and even indices differently. According to the cyclic sequence, there are one-third nanoporous graphene in our framework open a gap. The physical correspondence of Dirac point matching in reciprocal space and charge distribution in real space for the gap opening/closing is revealed. The periodic nanoperforation induced gap sizes were fitted as functions of three key indices. Furthermore, the coexistence of Dirac and flat bands is observed for some unique structures, which is sensitive to the atomic patterns. Our results indicate that the energy gap tunability of nano-perforated graphene can be used for flexible optoelectronic, photocatalytic and electrochemical applications. In particular, the atomically-thin electrically-connected geometry of nanoporous graphene suggests potential application in functional conductive coatings. The clear advantage of our system is in the combination of two properties: the possibility of applying electric voltage to the perforated sheet and rich chemically active atomic sites.

The AgInS 2 colloidal quantum dot (CQD) is a promising photoanode material with a relatively wide band gap for photoelectrochemical (PEC) solar-driven hydrogen (H 2) evolution. However, the unsuitable energy band structure still forms undesired energy barriers and leads to serious charge carrier recombination with low solar to hydrogen conversion efficiency. Here, we propose to use the ZnS shell for defect passivation and Cu ion doping for band structure engineering to design and synthesize a series of Cu x Ag 1−x InS 2 /ZnS CQDs. ZnS shell-assisted defect passivation suppresses charge carrier recombination because of the formation of the core/shell heterojunction interface, enhancing the performance of PEC devices with better charge separation and stability. More importantly, the tunable Cu doping concentration in AgInS 2 CQDs leads to the shift of the quantum dot band alignment, which greatly promotes the interfacial charge separation and transfer. As a result, Cu x Ag 1−x InS 2 /ZnS CQD photoanodes for PEC cells exhibit an enhanced photocurrent of 5.8 mA cm −2 at 0.8 V versus the RHE, showing excellent photoelectrocatalytic activity for H 2 production with greater chemical-/photostability.

Strain analysis has significance both for tailoring material properties and designing nanoscale devices. In particular, strain plays a vital role in engineering the growth thermodynamics and kinetics and is applicable for designing optoelectronic devices. In this paper, we present a methodology for establishing the relationship between elastic bond constants and measurable parameters, i.e., Poisson's ratio ν and systematic elastic constant K. At the atomistic level, this approach is within the framework of linear elastic theory and encompasses the neighbor interactions when an atom is introduced to stress. Departing from the force equilibrium equations, the relationships between ν, K, and spring constants are successfully established. Both the two-dimensional (2D) square lattice and common three-dimensional (3D) structures are taken into account in the procedure for facilitating, bridging the gap between structural complexity and numerical experiments. A new direction for unders...

Self-assembled submicron nickel particles were successfully synthesized via the one-step surfactant-assisted solvothermal method. The impact of surfactant and reducing agent stoichiometry is investigated in this manuscript. Different morphologies and structures of Ni particles, including flower-like nanoflakes, hydrangea-like structures, chain structures, sphere-like structures, and hollow structures were prepared through different processing conditions with two parameters such as temperature and time. Based on scanning electron microscopy (SEM), X-ray diffraction (XRD), thermal gravimetric analysis (TGA) and vibrating sample magnetometry (VSM), the submicron nickel particles show good saturation magnetization and excellent thermal stabilities with a possible growth mechanism for the variety of the structure-tuned formation. Importantly, the microwave absorption properties of the submicron nickel particles were studied. The lowest reflection loss of Ni-P₉/T/H with a thin layer thick...

A thickness dependent band gap is commonly found in layered two-dimensional (2D) materials. Here, using a C3N bilayer as a prototypical model, we systematically investigated the evolution of a band gap from a single layer to a bilayer using first principles calculations and tight binding modeling. We show that in addition to the widely known effect of interlayer hopping, de-charge transfer also plays an important role in tuning the band gap. The de-charge transfer is defined with reference to the charge states of atoms in the single layer without stacking, which shifts the energy level and modifies the band gap. Together with band edge splitting induced by the interlayer hopping, the energy level shifting caused by the de-charge transfer determines the size of the band gap in bilayer C3N. Our finding, applicable to other 2D semiconductors, provides an alternative approach for realizing band gap engineering in 2D materials.

In this paper, a framework of {w 1 , w 2 , R} classification for constructing the graphene nanomesh (GNM) of zigzag-edged hexagonal nanohole is systematically built. The three integer indexes w 1 , w 2 , and R indicate the distances between two neighboring sides of nanoholes in two directions and the nanohole size respectively, which leading to a straightforward gap opening criteria, i.e., 1 2 3 1, w w R n n + − = + ∈ , steered via DFT band structure calculations. The guiding rule indicates that the semimetallic and semiconducting variation is consistent with a peculiar sequence "010" and "100" ("0"/"1" represent gap closure/opening) with a period of 3 for odd and even w 1 respectively. The periodic nanoperforation induced gap sizes agreewith a linear fitting with a smaller � � ratio, while deviates from that when 1 2 () 1 w w R + < +. Particularly, the {p, 1, p} and {1, q, q} structures demonstrate each unique scaling rule pertaining to the nanohole size only when n is set to zero. Furthermore, the coexistence of Dirac and flat bands is observed for {1, q, q} and {1, 1, m} structures, which is sensitive to the atomic patters.

Carbon nitride compounds have emerged recently as a prominent member of 2D materials beyond graphene. The experimental realizations of 2D graphitic carbon nitride g-C 3 N 4 , nitrogenated holey grahpene C 2 N, polyaniline C 3 N have shown their promising potential in energy and environmental applications. In this work, we predict a new type of carbon nitride network with a C 9 N 4 stoichiometry from first principle calculations. Unlike common C-N compounds and covalent organic frameworks (COFs), which are typically insulating, surprisingly C 9 N 4 is found to be a 2D nodal-line semimetal (NLSM). The nodal line in C 9 N 4 forms a closed ring centered at  point, which originates from the p z orbitals of both C and N. The linear crossing happens right at Fermi level contributed by two sets of dispersive Kagome 2 and Dirac bands, which is robust due to negligible spin-orbital-coupling (SOC) in C and N. Besides, it is revealed that the formation of nodal ring is of accidental band degeneracy in nature induced by the chemical potential difference of C and N, as validated by a single orbital tight-binding model, rather than protected by crystal in-plane mirror symmetry or band topology. Interestingly, a new structure of nodal line, i.e., nodal-cylinder, is found in momentum space for AA-stacking C 9 N 4. Our results imply possible functionalization for a novel metal-free C-N covalent network with interesting semimetallic properties.

Massive efforts have been devoted to enhancing performances of Li-S batteries to meet the requirements of practical applications. However, problems remain in enhancing the energy density and improving the cycle life. We present a free-standing structure of walnut-shaped VS 4 nanosites combine with carbon nanotubes (NTs) as cathodes. In this framework, NT arrays provide high surface area and conductivity for high sulfur loadings, and VS 4 nanosites facilitate trapping and catalytic conversions of lithium polysulfides. The synergistic effects of free-standing NT arrays and VS 4 nanosites have enabled high rate capability up to 6 C and longterm cycling with a low decay rate of 0.037% up to 1200 cycles at 2 C. Moreover, the designed cathode can achieve high areal capacities up to ∼13 mAh•cm -2 and estimated gravimetric energy density of 243.4 Wh• kg -1 at a system level, demonstrating great potential in practical applications of Li-S batteries.

HIGHLIGHTS • The tunable mechanisms of lateral heterostructures on both homogeneous junctions and heterogeneous junctions are summarized. • Electronic and photoelectronic devices with lateral heterostructures have been discussed. • Different types of contacts of 2D lateral heterostructures are classified. • Recent developments in synthesis and nanofabrication technologies of 2D lateral heterostructures are reviewed. ABSTRACT Recent developments in synthesis and nanofabrication technologies offer the tantalizing prospect of realizing various applications from twodimensional (2D) materials. A revolutionary development is to flexibly construct many different kinds of heterostructures with a diversity of 2D materials. These 2D heterostructures play an important role in semiconductor and condensed matter physics studies and are promising candidates for new device designs in the fields of integrated circuits and quantum sciences. Theoretical and experimental studies have focused on both vertical and lateral 2D heterostructures; the lateral heterostructures are considered to be easier for planner integration and exhibit unique electronic and photoelectronic properties. In this review, we give a summary of the properties of lateral heterostructures with homogeneous junction and heterogeneous junction, where the homogeneous junctions have the same host materials and the heterogeneous junctions are combined with different materials. Afterward, we discuss the applications and experimental synthesis of lateral 2D heterostructures. Moreover, a perspective on lateral 2D heterostructures is given at the end.