Prof Laura Herz FRS

Abstract:

Optical Pump Terahertz Probe (OPTP) spectroscopy is a well-established measurement technique with which charge-carrier dynamics of semiconductor nanowires (NW) can be extracted in a noncontact manner. Here in this work, we employ OPTP spectroscopy for measuring temperature-dependent photoconductivity spectra of 3D Dirac Cd 3 As 2 semi-metal nanowires, revealing a high Extrinsic carrier concentration of ∼2.0x1017cm−3 and ultrahigh carrier mobility of up to ∼13x103cm2V−1s−1 at a temperature of 5 K.

Abstract:

Topological insulator (TI) materials are emerging as novel materials for spintronic applications. Here, we demonstrate helicity-dependent THz emission from Dirac semi-metal Cd 3 As 2 nanowires and used scattering-type scanning optical microscopy (s-SNOM) to identify potential single nanowire candidates for device applications. The preliminary investigation data of a candidate nanowire shows a homogenous topography and constant dielectric function in the MIR range. Indicating high-quality crystalline growth ideal for topological characterization.

Abstract:

Efficient charge-carrier transport is critical to the success of emergent semiconductors in photovoltaic applications. So far, disorder has been considered detrimental for charge-carrier transport, lowering mobilities and causing fast recombination. This work demonstrates that, when properly engineered, cation disorder in a multinary chalcogenide semiconductor can considerably enhance the charge-carrier mobility and extend the charge-carrier lifetime. Here, the properties of AgBiS₂ nanocrystals (NCs) are explored where Ag and Bi cation-ordering can be modified via thermal-annealing. Local Ag-rich and Bi-rich domains formed during hot-injection synthesis are transformed to induce homogeneous disorder (random Ag-Bi distribution). Such cation engineering results in a six-fold increase in the charge-carrier mobility, reaching ∼2.7 cm²V⁻¹s⁻¹ in AgBiS₂ NC thin films. It is further demonstrated that homogeneous cation disorder reduces charge-carrier localisation, a hallmark of charge-carrier transport recently observed in silver-bismuth semiconductors. This work proposes that cation-disorder engineering flattens the disordered electronic landscape, removing tail states that would otherwise exacerbate Anderson localisation of small polaronic states. Together, these findings unravel how cation-disorder engineering in multinary semiconductors can enhance the efficiency of renewable energy applications.

Abstract:

Topological materials have rapidly gained interest as contenders for development of coherent, controllable terahertz emitters. Possessing Weyl nodes either at the surface or within the bulk, they host spin-polarised, helicity-dependent currents that offer possibility to control the emitted THz pulse by changing the polarization of the optical pulses generating the radiation. Here, we show that upon near-infrared excitation at oblique incidence, multi-cycle pulses are generated with a narrow bandwidth of ∼0.4 THz for cadmium arsenide bulk crystals and nanowire ensembles. Both the bandwidth and peak emission frequency of the generated THz radiation can be tuned by respectively varying the photon helicity and angle of incidence of the photoexcitation light.