Charge Carrier Trapping and Acoustic Phonon Modes in Single CdTe Nanowires

Abstract

Semiconductor nanostructures produced by wet chemical synthesis are extremely heterogeneous, which makes single particle techniques a useful way to interrogate their properties. In this paper the ultrafast dynamics of single CdTe nanowires are studied by transient absorption microscopy. The wires have lengths of several micrometers and lateral dimensions on the order of 30 nm. The transient absorption traces show very fast decays, which are assigned to charge carrier trapping into surface defects. The time constants vary for different wires due to differences in the energetics and/or density of surface trap sites. Measurements performed at the band edge compared to the near-IR give slightly different time constants, implying that the dynamics for electron and hole trapping are different. The rate of charge carrier trapping was observed to slow down at high carrier densities, which was attributed to trap-state filling. Modulations due to the fundamental and first overtone of the acoustic breathing mode were also observed in the transient absorption traces. The quality factors for these modes were similar to those measured for metal nanostructures, and indicate a complex interaction with the environment.

CdTe Nanowires studied by Transient Absorption Microscopy

“…The measured periods are in good agreement with the results of a continuum model [15], confirming the assignment. In these calculations, the bulk elastic constants of CdTe were used [16], and the NW radius was estimated from the optical absorption cross-section measurements [5], The damping of the oscillations is due to radiation of sound waves into the surrounding medium [17]. The quality factors measured for the CdTe NWs ( Q = πτ T ) are similar to those for Ag NWs and nanocubes measured in our laboratory [18].…”

“…[5]. The ensemble linear and transient absorption (TA) spectra of the CdTe NW sample are shown in Fig.…”

Imaging nano-objects by linear and nonlinear optical absorption microscopies

“…The fast 7 ps decay in this trace is attributed to internal relaxation, and the slower nanosecond decay that appears as an offset in these experiments is assigned to energy transfer from the particle to its surroundings (in this case the glass substrate). TAM experiments can also be used to study charge carrier relaxation in semiconductor nanostructures, [21,45,46,55,83,97,109,110,134,137,145,146,164] and conducting polymers. [52,[171][172][173] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 between different nanostructures, and also about spatial variations within a single nanostructure.…”

“…The past decade has seen tremendous activity in the development of optical 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 dynamics of the system being studied, such as the time scales for charge carrier trapping and diffusion, [21,45,46,49,55,62,72,97,109,110,145,171] and how the materials dissipate energy into the environment. [20,101,133,152,153,179] However, these experiments are difficult to implement and require complex ultrafast laser systems.…”

Optical Control of Internal Electric Fields in Band Gap-Graded InGaN Nanowires

“…The saturation can be explained by a bleaching effect by state filling at the band edges. [34][35][36] Although we have not explicitly included the effect of In-rich clusters in our energy band modeling, it is speculated that such clusters would create local potential minima that capture photo-generated carriers and enhance the recombination. As a result, if the number of In-rich clusters were increased, then the minority carrier lifetime for both electrons and holes would be expected to decrease, lowering the photocurrent.…”