Ultrafast Photoconductivity and Terahertz Vibrational Dynamics in Double‐Helix SnIP Nanowires

Abstract

SnIP could be the first of a new class of inorganic double-helix materials. [7][8][9] With strong intra-helix covalent bonds and weak inter-helix dispersion forces, SnIP belongs to the group of newly emerging 1D van der Waals (vdW) materials with potential applications in nanoelectronics and photonics. [10][11][12][13] In contrast to the DNA structure, which consists of two equal radius helices, SnIP forms with an outer [SnI] + helix wrapping around an inner [P] − helix, as pictured in Figure 1a. SnIP crystallizes monoclinically with a unit cell containing two opposite-handed double helices so that there is no net chirality. It is composed of abundant and non-toxic elements and can grow uninhibited to cm-length needles with a low-temperature synthesis [6,14] (Sections S1 and S2, Supporting Information) or in nanotubes using vapor deposition. [15,16] Its 1.86 eV band gap, as determined by band structure calculations (Figure 1b,c) and verified experimentally (see ref.[ 6 ] and Section S3, Supporting Information), is well situated for solar absorption and photocatalytic water splitting. [8,10,15] SnIP is also an extremely soft and flexible semiconductor and is therefore a promising material for applications in flexible electronics, [6,10] where these properties are highly desirable. [17] It is predicted to have a high carrier mobility; [8] however, as-grown SnIP is highly resistive so that the current lack of doped samples has made it difficult to explore its electronic properties. [6] Moreover, despite the exciting properties and unique structure of SnIP, there have been no investigations probing its ultrafast photophysical properties.Here, we use time-resolved terahertz (THz) spectroscopy (TRTS) to study picosecond charge carrier dynamics in SnIP nanowire films, as shown in Figure 1d. TRTS, a powerful non-contact ultrafast probe, has been used extensively to probe carrier dynamics in low-dimensional materials, accelerating scientific understanding of transport mechanisms and enabling materials optimization for potential applications. [18,19] From analysis of the photoconductivity spectra, along with insight into the highly anisotropic energy landscape from density functional theory (DFT), we make the first measurement of the carrier mobility in SnIP. We find a maximum electron mobility of 280 cm 2 V −1 s −1 along the double-helix axis, an extraordinarily high mobility for a material as soft and flexible as SnIP. On Tin iodide phosphide (SnIP), an inorganic double-helix material, is a quasi-1D van der Waals semiconductor that shows promise in photocatalysis and flexible electronics. However, the understanding of the fundamental photophysics and charge transport dynamics of this new material is limited. Here, time-resolved terahertz (THz) spectroscopy is used to probe the transient photoconductivity of SnIP nanowire films and measure the carrier mobility. With insight into the highly anisotropic electronic structure from quantum chemical calculations, an electron mobility as high as 280 cm 2 V −1 s −1 along the doub...

Chirality-Induced Propagation Velocity Asymmetry

“…33 Time-resolved terahertz (THz) spectroscopy revealed a high intrinsic electron mobility along the double-helix axis, which is limited by traps. 35 Individual SnIP double-helix strands can be obtained from the racemic mixture by minimizing the van de Waals force and stabilizing the single strands in a different medium, such as in SnIP@C 3 N 4 (F,Cl) thin films. 33 Another possibility, as indicated by total energy calculations, 33,36 is the encapsulation of individual strands in carbon nanotubes.…”

“…In addition, the material is highly flexible and stable, showing polymer-like behavior . Time-resolved terahertz (THz) spectroscopy revealed a high intrinsic electron mobility along the double-helix axis, which is limited by traps . Individual SnIP double-helix strands can be obtained from the racemic mixture by minimizing the van de Waals force and stabilizing the single strands in a different medium, such as in SnIP@C 3 N 4 (F,Cl) thin films .…”

“…These results can be summarized as follows: (1) there is no observable difference for spinless wavepackets traveling along CW or CCW directions on M (Figure 2c) or P-SnIP (Figure S2) strands; (2) the continuity equation derived for the TB formalism is consistent with the quantum dynamics calculations (Figure 2d); (3) the time of flight the wavepackets take to travel 45 unit cells (≈35.7 nm) in the absence of SOC is τ tof = 109 fs; and ( 4) by equating the LHS with RHS of eq 15 we obtain a tight-binding effective mass of m* ≈ 0.42 m e , which is similar to the value of 0.28 m e that was obtained from ab initio band structure calculations. 35 Thus, ignoring the SOC, the results for M-SnIP and P-SnIP strands show no observable difference (Figure S2).…”

Inorganic SnIP-type double helices: promising candidates for high-efficiency photovoltaic cells

“…The primitive cell of SnIP contains a le-and a right-handed double helix bonded by weak van der Waals (vdW) interactions, with each double helix having a small [P] helix nested into the relatively larger [SnI] helix. The unique structure makes SnIP show both interesting electronic properties such as high carrier mobility as a typical inorganic semiconductor 18,19 and distinctive mechanical properties such as high exibility, deformability, and especially a remarkably low Young's modulus (13.6 GPa) as a stable inorganic material. 20 Recent studies revealed that SnIP also has special topological properties which can exhibit topological nodal rings/ lines in both the bulk phonon modes and their corresponding surface states.…”

Nanotube Matrices for Flexible SnIP Nanowires

“…The formation mechanism and stacking motives have been published recently, , while the effect on intra- and interhelical bonding relations and their influence on each other after hybrid material formation are still somewhat uncertain. Recently, time-resolved terahertz spectroscopy was used to investigate the transient photoconductivity and charge carrier mobility in SnIP . An electron mobility of 280 cm 2 V –1 s –2 along and a hole mobility of 238 cm 2 V –1 s –2 perpendicular to the SnIP double helices were detected.…”