Session N13: Materials for Applied Physics

Unraveling the Radiative Cooling Properties of Inexpensive, Ubiquitous and Biodegradable Cellulose Materials.

Radiative cooling is a passive cooling mechanism that enables objects to dissipate heat by emitting infrared radiation to the cold expanse of outer space. In this presentation, we will unveil the latest findings from our research into the radiative cooling capabilities of cellulose materials. Our investigations have revealed the remarkable potential of specific cellulose composites to achieve substantial radiative cooling effects, with temperature differentials as impressive as 15 degrees Celsius below ambient conditions without using any external energy. Moreover, we have conducted comprehensive measurements to quantify the radiative cooling power exhibited by these cellulose composites and naturally occurring biodegradable cellulose.

The applications of this research extend beyond theoretical understanding. These cellulose-based radiative cooling materials hold promise for a wide range of practical applications. From enhancing the thermal comfort of clothing in hot and humid climates to contributing to energy-efficient building materials and cooling technologies, the possibilities are vast. We would explore cellulose-based radiative cooling its potential to revolutionize sustainable cooling technologies without contributing to global warming and improve the quality of life in various environmental conditions.   

Gecko-inspired adhesive structures for amphibious soft robot locomotion.

Over the years, efforts in bioinspired soft robotics have led to mobile systems that emulate features of natural animal locomotion. This includes combining mechanisms from multiple organisms to further improve movement. In this work, we seek to improve locomotion in soft, amphibious robots by combining two independent mechanisms: sea star locomotion gait and gecko adhesion. Specifically, we aim to test and compare various microstructures using different soft polymers to determine the optimal material with the corresponding microstructure configuration. We tested hemispherical, cylindrical and wedge-shaped microstructures made of PDMS and polyurethane to determine the adhesion on glass, acrylic, wood and metal surfaces. We determined the optimal geometric configuration for each type of microstructure by mathematically modeling the adhesion response of various geometric configurations. The gecko-inspired adhesives were subsequently subjected to experimentation on a compliant, pneumatically actuated limb, intended for integration into a soft robot inspired by sea stars. These adhesives, drawing inspiration from gecko adhesive structures, demonstrated a significant augmentation in adhesion properties across different substrates and enabled the robot to ascend inclines with a steepness of up to 25° and to maintain a stable grip on slopes inclined at 51° ± 6° under static conditions.   

Highly sensitive photodetector based on multijunction 2D-lateral heterostructures

The accessibility of various 2D materials facilitates the fabrication of many atomically thin devices, with a strong motivation towards manipulating quantum light detection. Unlike vertical 2D heterostructures, spatially separated 2D transition metal dichalcogenides (TMDs) lateral heterostructures could offer an exciting platform for probing carrier dynamics of excitons, trions, and their applications in optoelectronics. In this work, we demonstrate CVD-grown multijunction 2D MoSe₂-WSe₂ lateral heterostructures as spectrally tunable photosensors. The photoresponse characteristics of these 2D- phototransistors and other critical device parameters such as responsivity, gain, effective detectivity, and efficiency are evaluated using a tunable laser source in the visible to near-infrared regime. Structural-optical-electrical results are correlated to their macroscale photo-sensing characteristics for efficient photodetector design. This information will enhance the potential in the fields of low-powered electronics, non-volatile memory, and tunable quantum detectors based on lateral heterostructures.   

Orthorhombic CsPbI3 Inorganic Perovskite: DFT Insights into Its Mechanical and Optical Properties

We present Density Functional Theory (DFT) study of the multifaceted properties of CPbI₃, an inorganic perovskite semiconductor, with a particular focus on its potential in photovoltaic applications. We systematically analyze the interplay between its structural, mechanical, electronic, and optical characteristics, considering their relevance to solar cell technology.  Utilizing the linearized extended plane wave method (FP-LAPW) as implemented in Wien2K, we conduct a comparative assessment of our findings, obtained through distinct DFT approximation methods, including the local density approximation (LDA), generalized gradient approximation (GGA), and the modified Becke–Johnson potential (mBJ). Our investigation establishes that the orthorhombic phase is energetically most favorable compared to the cubic phase (convex Hull is ~0.03eV/atom), while both phases demonstrate mechanical stability and synthetic feasibility based on elastic tensor and lattice vibration calculation. Additionally, the band structure reveals the presence of an indirect bandgap of 1.2eV (GGA) that is increases to 2.1eV in mBJ calculations. Furthermore, we calculate and interpret essential optical parameters such as optical conductivity, index of refraction, dielectric constant. Our comprehensive analysis leads us to the conclusion that both the cubic and orthorhombic phases of CsPbI₃ hold substantial promise as optoelectronic materials that warrant further research and development.   

Phonon dispersion and its implications towards thermoelectricity

Phonons are quasi-particles that aid in the transfer of heat within solids, and their dynamics are essential for deriving various thermal properties like thermal expansion, specific heat, and lattice thermal conductivity. Materials exhibiting constituent elements with competitive atomic masses are responsible for the interaction between phonon modes, as supported by the phonon dispersion of simple structural forms [1] to the complex ones like skutterudites [2]. Highly interactive phonon modes affect thermal transport and eventually result in low lattice thermal conductivity, a vital thermoelectric coefficient. The phonon dispersion of various materials falling in the family of layered oxy-chalcogenides [3], metal-organic hybrid perovskites like CH₃NH₃SnI₃ [4] and oxides such as CsAgO [5] have been addressed here in light of significance of structural geometry upon lattice thermal transport. The comparative analysis of phonon dispersion for opted materials suggests that low-frequency phonon modes are highly interacting especially in CH₃NH₃SnI₃ and CsAgO, which is a promising sign for low lattice thermal transport in these materials and enable them viable candidates for good thermoelectrics.   

Effects of Different Interatomic Potentials on Fracture Simulations of Single Crystal Silicon

Since the advances in modern high-technology, silicon has attracted the attention of many researchers because of its variety of applications from semiconductors to biomedical implants. However, some gaps between experimental and simulation studies still exist, especially in those properties related to the failure behaviors of silicon. Atomistic simulation is one of the most suitable tools to understand the exact nature of silicon, but the outcomes are strongly tied to a particular interatomic potential used in the simulation. In the present work, fracture simulations of single crystal silicon are performed using three interatomic potential models: (1) Stillinger-Weber (SW), (2) Modified Embedded Atom Model (MEAM), and (3) ReaxFF. The simulations are repeated in five different crystallographic orientations and with various initial crack sizes as well as the different model sizes. The simulation results reveal that the crystal fails either through a slip deformation along the (111) plane or through a crack propagation in the perpendicular direction to the loading. It is seen that these failure behaviors depend on the various input parameters as well as the interatomic potential models. SW silicon model is the most ductile among the tested potential models, but it also shows the brittle fracture in most orientations. The mechanisms leading to the different failure behaviors are discerned and the failure behavior is predicted using the material parameters computed with each potential model.   

Predictions of Novel Polymorphs of Boron Nitride: A First-principles Study

Physical properties of Boron Nitride (BN) have been studied in novel crystal structures such as hexagonal (h), wurtzite (w), 5-5, GeP, Li₂O₂, MoC, NiAs, and TiAs. The calculations of structural, electronic, and optical properties of BN have been carried out by “the full-potential linearized augmented plane wave plus local orbital (FP-LAPW + lo)” method structured within the “density functional theory (DFT)”. The phonon band structures have been determined using the pseudo-potential-based approach realized in the CASTEP code, indicating that the h, w, 5-5, Li₂O₂, and MoC do not exhibit phonon modes at negative frequency, whereas GeP, NiAs, and TiAs modifications exhibit phonon modes at the negative frequency. However, the novel polymorphs of BN demonstrated cohesive energies higher/comparable to that of the ground state h-BN. The lattice parameters of h and w structures of BN calculated through “Perdew-Burke-Ernzerhof – generalized gradient approximation (PBE–GGA)” are in good agreement with the available theoretical and experimental data. The band structures calculations indicate that BN crystallized in h, w, GeP, Li₂O₂, MoC, NiAs, and TiAs show indirect bandgap, whereas the 5-5 phase shows direct bandgap. The bandgap values show that h-BN and w-BN are insulators, and 5-5, GeP, Li₂O₂, MoC, NiAs, and TiAs are semiconductors. Optical parameters, such as the real part of the dielectric function, the imaginary part of the dielectric, reflectivity, absorption coefficients, and refraction spectrum related to all the considered polymorphs, have been studied. These novel polymorphs with greatly evolved physical behaviour would be interesting for applications in the current semiconducting industry and other futuristic technologies.    

Fully consistent ab initio calculation of the shift current at steady state

The bulk photovoltaic effect (BPVE) is a process through which a homogeneous sample of a noncentrosymmetric material can generate a photocurrent. The effect is usually described in terms of two mechanisms: the ballistic current, where asymmetric scattering leads to unequal carrier generation rates at opposite momenta, and the shift current, where carrier excitation is accompanied by a net displacement in real space. However, the story does not stop there: after excitation, photoexcited carriers relax and recombine, forming a kinetic cycle at steady state. The relaxation and recombination processes are also predicted to produce photocurrent through a shift current mechanism, but so far, they have only been calculated in model systems. Thus, it is difficult to know the relative magnitude of the relaxation currents in comparison to the more commonly calculated excitation currents. To address this gap, we model photocurrents from the remainder of the kinetic cycle from first principles. In conjunction with existing ab initio descriptions of excitation shift and ballistic current, this allows us to make direct comparisons between different photocurrent contributions in real materials under realistic experimental conditions.   

g-factor Engineering in n-type InAsP alloys

InAs_xP_1-x offers a wide tunability of g-factor ranging from 1.2 for InP (x=0.0) to -14.75 for InAs (x=1.0) including a possible zero g-factor.  In this work, we studied the magneto-optical properties of InAs_xP_1−x films at room temperature and focused on two alloy compositions (x = 0.07 and 0.34). When it comes to photodetectors for quantum information and sensing, to preserve the entanglement it is important to fabricate devices using a material that has a conduction band effective g-factor much smaller than the valence band so that the photodetector can excite equally to the spin split states. In this talk, we present our experimental and theoretical results showing that a g-factor close to zero can be achieved in InAs_xP_1−x with the right alloy concentration (x~0.34). This fact introduces the prospects of this materials system regarding quantum communication devices and g-factor engineering.   

Doping and temperature dependence of optical conductivity of Nb(x)V(1-x)O2 single crystals

Vanadium dioxide (VO2) is known for its reversible transformation from an insulating monoclinic phase to a metallic rutile phase, occurring just above room temperature. This property enables various photonics applications like optical switching and modulators. By adjusting the concentration of substituents through chemical doping, it becomes possible to manipulate the transition temperature, thereby broadening the potential applications of VO2 to lower energy scales. Here, we investigate the doping dependence of the optical conductivity of Nb(x)V(1-x)O2 single crystals (x = 0.05, 0.11, 0.14, 0.24, 0.35, and 0.88) at room temperature. We measured a reflection spectrum over a wide range of frequencies for the Kramers-Kronig analysis. With the small doping of Nb, the insulator-to-metal transition temperature is significantly reduced. The Drude optical conductivity shows up at 11% Nb at room temperature, indicating the transition temperature is lower than room temperature. We also performed the temperature dependence of the optical conductivity, especially for 11% Nb. The insulator-to-metal transition shows around -45 degrees Celsius. We applied an extended Drude model to fit the optical conductivity to evaluate the metal-to-insulator or the metal-to-semiconductor transition.