We report ultrafast transient-grating measurements of crystals of the three-dimensional Dirac semimetal cadmium arsenide, Cd 3 As 2 , at both room temperature and 80 K. After photoexcitation with 1.5-eV photons, charge-carriers relax by two processes, one of duration 500 fs and the other of duration 3.1 ps. By measuring the complex phase of the change in reflectance, we determine that the faster signal corresponds to a decrease in absorption, and the slower signal to a decrease in the light's phase velocity, at the probe energy. We attribute these signals to electrons' filling of phase space, first near the photon energy and later at lower energy. We attribute their decay to cooling by rapid emission of optical phonons, then slower emission of acoustic phonons. We also present evidence that both the electrons and the lattice are strongly heated.

We demonstrate an intrinsically fast, Seebeck-type metal-semimetal-metal infrared photodetector based on Cd 3 As 2 crystals. The Seebeck voltage is induced under off-center illumination, leading to asymmetric temperature gradients and net current flow.The sensor's room-temperature responsivity is 0.27 mA/W. The photocurrent signal is readily registered at a modulation frequency of 6 kHz, and the sensor's intrinsic bandwidth is predicted to approach terahertz. We find that the photocurrent depends on the optical power and modulation frequency. Our study reveals crystalline Cd 3 As 2 as a promising material for high-bandwidth and spectrally broad photo-sensing, imaging, and communication.

Patterning graphene with noble metal plasmonic nanostructures to enhance and to manipulate the optical and electronic properties of graphene promises a variety of technological innovations in the field of nano-optoelectronics. In this report, we briefly revisit photoconduction experiments done recently on graphene plasmonic sensors and show that the excess electrical current generated in response to spatially non-uniform optical excitation is primarily induced by a photo-thermo-electric effect in the graphene itself. As this mechanism has nothing to do with the excess free carrier generation common with conventional semiconductors, the plasmonic nanostructures cannot be utilized to regulate the free carrier density and doping of graphene, in contrast to what has been proposed recently.

carrier back-scattering remains suppressed and the electrical transport is dominated by high-energy carriers that favorably affect the thermopower of Cd 3 As 2 . [ 9 ] Since the power factor is to increase with the carrier mobility, µ , and weighted density-ofstates effective mass, i.e., S 2 σ ≈ µ D *1.5 m , and the lattice thermal conductivity is predicted to be very low, Cd 3 As 2 shows a strong potential for demonstrating high, device-favorable S , and in turn ZT , Table 1 . [ 10 ] Binary cadmium arsenide that solidifi es as Cd 3 As 2 is known to undergo multiple polymorphic solid-solid transitions β α α α → ′′ → ′ → at T of 578, 475, and 225 °C, respectively; the latter of which limits the growth temperature to 225 °C. Given the melting point of this compound of 721 °C, melt-based crystal growth techniques have to be ruled out from consideration. Pulsed-laser evaporation was offered as an alternative route, [ 17 ] but the resultant samples show highly reduced, i.e., polycrystalline and amorphous quality.In this work, a low-T chemical-vapor deposition route is used instead to grow platelets up to cm size in Cd 3 As 2 in a horizontally oriented hot-wall atmospheric pressure chemical vapor deposition (CVD) reactor. To lower the residual pressure of oxygen and water vapors, the reactor was initially fl ushed several times by bringing the base pressure down to ≈1 m Torr and refi lling it with 99% pure argon gas. The temperature of the furnace was gradually ramped up from ≈25 to 700 °C (measured at the middle point of the quartz tube) within an hour. Pure, polycrystalline Cd 3 As 2 chunks were used as a precursor, while quartz and alumina wafers served as growth substrates. To facilitate vapor transport of Cd and As, a steady argon fl ow was maintained within the reactor with the rate of ≈0.2 SCCM. The growth duration was set not to exceed 3 h. Upon completion, the heater was shut down and the reactor was left to cool down naturally at the average rate of ≈5 °C min −1 in the presence of argon fl ow. Except for the tube end points, the temperature gradient (as probed with the help of IR temperature sensor) along the tube main axis, T ∇ was on average a few degrees per cm, Figure 1 , top.After the growth, the substrate as well as the reactor sidewalls was subject to visual and optical microscope inspections, Thermoelectric generators (TEGs) directly convert thermal into electrical energy and are the prime candidates for application in low-grade thermal energy/heat waste recovery. These solidstated devices demonstrate several critical advantages over other power generators as they are silent, very reliable, and compact/lightweight, which make them attractive for several niche deployments including transportation, space satellites, electronics cooling, and thermovoltaic cells. [1][2][3] Yet, TEGs with typical effi ciency of only ≈5%-8% demonstrate poor effi ciencyto-cost ratio, and the problem stems mainly from the poor conversion characteristics of the thermoelectric (TE) materials themselves.The TE performance is rou...

This work presents the concept of a monolithic concrete-integrated dye-synthesized photovoltaic solar cell for optical-to-electrical energy conversion and on-site power generation. The transport measurements carried out in the dark revealed the presence of VOC of ~190 mV and ISC of ~9 μA, induced by the electrochemical conversion of concrete-supplied ionic impurities at the electrodes. The current-voltage measurements performed under illumination at incident optical powers of ~46 mW confirmed the generation of electrical power of ~0.64 μW with almost half generated via battery effect. This work presents a first step towards realizing the additional pathways to low-cost electrical power production in urban environments based on a combined use of organic dyes, nanotitania and concrete technology.