Photogeneration and Mobility of Charge Carriers in Atomically Thin Colloidal InSe Nanosheets Probed by Ultrafast Terahertz Spectroscopy

Abstract

The implementation of next generation ultrathin electronics by applying highly promising dimensionality-dependent physical properties of two-dimensional (2D) semiconductors is ever increasing. In this context, the van der Waals layered semiconductor InSe has proven its potential as photodetecting material with high charge carrier mobility. We have determined the photogeneration charge carrier quantum yield and mobility in atomically thin colloidal InSe nanosheets (inorganic layer thickness 0.8–1.7 nm, mono/double-layers, ≤ 5 nm including ligands) by ultrafast transient terahertz (THz) spectroscopy. A near unity quantum yield of free charge carriers is determined for low photoexcitation density. The charge carrier quantum yield decreases at higher excitation density due to recombination of electrons and holes, leading to the formation of neutral excitons. In the THz frequency domain, we probe a charge mobility as high as 20 ± 2 cm2/(V s). The THz mobility is similar to field-effect transistor mobilities extracted from unmodified exfoliated thin InSe devices. The current work provides the first results on charge carrier dynamics in ultrathin colloidal InSe nanosheets.

Ultrafast Transient Absorption and Terahertz Spectroscopy as Tools to Probe Photoexcited States and Dynamics in Colloidal 2D Nanostructures

“…The lateral size of the nanosheets increases from 300-800 nm for octadecylamine as a ligand to 1-2 μm when changing the ligand to decylamine [7,16]. InSe nanosheets show weak photoluminescence near 600 nm [16], assumedly originating from the slightly indirect band gap of mono/double InSe layers as predicted by DFT calculations [7]. TA and THz spectroscopy are used …”

“…TA is sensitive to the sum of the electron and hole population densities and thus probes the sum of free charge carriers and Coulombically bound excitons, but it lacks the ability to distinguish between these species. For disentangling the contribution of excitons and/or free charges in a photoexcited sample and to determine the sample's "intrinsic" conductivity/ mobility, THz spectroscopy can be used [7]. Figure 4 shows a scheme of the THz set-up.…”

“…The frequency dependent conductivity due to charges is determined by the field-induced drift velocity in the oscillating THz electric field E(t) = E 0 cos(ω,t). The complex valued mobility of charges and excitons can be related to three different velocity components in the THz field: (1) The velocity of free charges in- phase with the THz field (see Figure 4) v(t) = μ R (ω)E 0 cos(ω,t) with μ R (ω) the real component of the charge mobility, (2) the velocity of free charges out-of-phase with the THz field v(t) = μ I (ω)E 0 sin(ω,t), with μ I (ω) the imaginary component of the charge mobility, and (3) the relative out-of-phase velocity of an electron and hole that are Coulombically bound v eh (t) = μ I (ω)E 0 sin(ω,t) with ( ) I e αω µ ω = and α the polarizability of the exciton (see Figure 4) [7]. From the measurements we obtain the differential transmission of the THz waveform ΔE(t p ,t) = E excited (t p ,t)-E 0 (t p ) with E excited (t p ,t), the transmitted THz electric field after photoexcitation of the sample and E 0 (t p ), the transmitted THz electric field in absence of photoexcitation (t p is the time between generating and detecting the THz field and t the time delay between the photoexcitation pump pulse and the THz probe pulse).…”

“…Note that equation (3) does not require the actual sample thickness L, but only the photoexcitation density N a per unit area [7]. Figure 5a depicts the quantity S(t) = Φ e (t)μ e + Φ h (t)μ h , as a function of time, t, after photoexcitation, which is the sum of the product of the quantum yield of electrons, Φ e (t) and holes, Φ h (t), and the real mobility component of their mobility, (μ e and μ h ).…”

“…We discuss recent TA and THz measurements on ultrathin 2D InSe nanosheets, a material with highly interesting optoelectronic properties that also underpin the general applicability of both methods for an in-depth spectroscopic characterization of 2D semiconductors [7].…”