skip to main content


Title: Stacking‐Order‐Driven Optical Properties and Carrier Dynamics in ReS 2
Abstract

Two distinct stacking orders in ReS2are identified without ambiguity and their influence on vibrational, optical properties and carrier dynamics are investigated. With atomic resolution scanning transmission electron microscopy (STEM), two stacking orders are determined as AA stacking with negligible displacement across layers, and AB stacking with about a one‐unit cell displacement along theaaxis. First‐principles calculations confirm that these two stacking orders correspond to two local energy minima. Raman spectra inform a consistent difference of modes I & III, about 13 cm−1for AA stacking, and 20 cm−1for AB stacking, making a simple tool for determining the stacking orders in ReS2. Polarized photoluminescence (PL) reveals that AB stacking possesses blueshifted PL peak positions, and broader peak widths, compared with AA stacking, indicating stronger interlayer interaction. Transient transmission measured with femtosecond pump–probe spectroscopy suggests exciton dynamics being more anisotropic in AB stacking, where excited state absorption related to Exc. III mode disappears when probe polarization aligns perpendicular tobaxis. The findings underscore the stacking‐order driven optical properties and carrier dynamics of ReS2, mediate many seemingly contradictory results in the literature, and open up an opportunity to engineer electronic devices with new functionalities by manipulating the stacking order.

 
more » « less
Award ID(s):
1707080 1720595
NSF-PAR ID:
10457993
Author(s) / Creator(s):
 ;  ;  ;  ;  ;  ;  ;  ;  ;  ;  ;  
Publisher / Repository:
Wiley Blackwell (John Wiley & Sons)
Date Published:
Journal Name:
Advanced Materials
Volume:
32
Issue:
22
ISSN:
0935-9648
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    We performed polarized reflection and transmission measurements on the layered conducting oxide PdCoO2thin films. For theab-plane, an optical peak near Ω ≈ 750 cm−1drives the scattering rate 1/τ(ω) and effective massm*(ω) of the Drude carrier to increase and decrease respectively forω ≧ Ω. For thec-axis, a longitudinal optical phonon (LO) is present at Ω as evidenced by a peak in the loss function Im[−1/εc(ω)]. Further polarized measurements in different light propagation (q) and electric field (E) configurations indicate that the Peak at Ω results from an electron-phonon coupling of theab-plane carrier with thec-LO phonon, which leads to the frequency-dependent 1/τ(ω) andm*(ω). This unusual interaction was previously reported in high-temperature superconductors (HTSC) between a non-Drude, mid-infrared (IR) band and ac-LO. On the contrary, it is the Drude carrier that couples in PdCoO2. The coupling between theab-plane Drude carrier andc-LO suggests that thec-LO phonon may play a significant role in the characteristicab-plane electronic properties of PdCoO2, including the ultra-high dc-conductivity, phonon-drag, and hydrodynamic electron transport.

     
    more » « less
  2. Metalorganic chemical vapor deposition (MOCVD) growths of β-Ga 2 O 3 on on-axis (100) Ga 2 O 3 substrates are comprehensively investigated. Key MOCVD growth parameters including growth temperature, pressure, group VI/III molar flow rate ratio, and carrier gas flow rate are mapped. The dependence of the growth conditions is correlated with surface morphology, growth rate, and electron transport properties of the MOCVD grown (100) β-Ga 2 O 3 thin films. Lower shroud gas (argon) flow is found to enhance the surface smoothness with higher room temperature (RT) electron Hall mobility. The growth rate of the films decreases but with an increase of electron mobility as the VI/III molar flow rate ratio increases. Although no significant variation on the surface morphologies is observed at different growth temperatures, the general trend of electron Hall mobilities are found to increase with increasing growth temperature. The growth rates reduce significantly with uniform surface morphologies as the chamber pressure increases. By tuning the silane flow rate, the controllable carrier concentration of (100) β-Ga 2 O 3 thin films between low-10 17  cm −3 and low-10 18  cm −3 was achieved. Under optimized growth condition, an (100) β-Ga 2 O 3 thin film with RMS roughness value of 1.64 nm and a RT mobility of 24 cm 2 /Vs at a carrier concentration of 7.0 × 10 17  cm −3 are demonstrated. The mobilities are primarily limited by the twin lamellae and stacking faults defects generated from the growth interface. Atomic resolution scanning transmission electron microscopy reveals the formation of twin boundary defects in the films, resulting in the degradation of crystalline quality. Results from this work provide fundamental understanding of the MOCVD epitaxy of (100) β-Ga 2 O 3 on on-axis Ga 2 O 3 substrates and the dependence of the material properties on growth conditions. The limitation of electron transport properties of the (100) β-Ga 2 O 3 thin films below 25 cm 2 /Vs is attributed to the formation of incoherent boundaries (twin lamellae) and stacking faults grown along the on-axis (100) crystal orientation. 
    more » « less
  3. Abstract

    A comprehensive experimental study on optical properties and photocarrier dynamics in Bi2O2Se monolayers and nanoplates is presented. Large and uniform Bi2O2Se nanoplates with various thicknesses down to the monolayer limit are fabricated. In nanoplates, a direct optical transition near 720 nm is identified by optical transmission, photoluminescence, and transient absorption spectroscopic measurements and is attributed to the transition between the valence and conduction bands in the Γ valley. Time‐resolved differential reflection measurements reveal ultrafast carrier thermalization and energy relaxation processes and a photocarrier recombination lifetime of about 200 ps in nanoplates. Furthermore, by spatially resolving the differential reflection signal, a photocarrier diffusion coefficient of about 4.8 cm2s−1is obtained, corresponding to a mobility of about 180 cm2V−1s−1. A similar direct transition is also observed in monolayer Bi2O2Se, suggesting that the states in the Γ valley do not change significantly with the thickness. The temporal dynamics of the excitons in the monolayer is quite different from the nanoplates, with a strong saturation effect and fast exciton–exciton annihilation at high densities. Spatially and temporally resolved measurements yield an exciton diffusion coefficient of about 20 cm2s−1.

     
    more » « less
  4. Abstract

    The flexible, transparent, and low‐weight nature of ferroelectric polymers makes them promising for wearable electronic and optical applications. To reach the full potential of the polarization‐enabled device functionalities, large‐scale fabrication of polymer thin films with well‐controlled polar directions is called for, which remains a central challenge. The widely exploited Langmuir–Blodgett, spin‐coating, and electrospinning methods only yield polymorphous or polycrystalline films, where the net polarization is compromised. Here, an easily scalable approach is reported to achieve poly(vinylidene fluoride‐trifluoroethylene) P(VDF‐TrFE) thin films composed of close‐packed crystalline nanowires via interface‐epitaxy with 1T′‐ReS2. Upon controlled thermal treatment, uniform P(VDF‐TrFE) films restructure into about 10 and 35 nm‐wide (010)‐oriented nanowires that are crystallographically aligned with the underlying ReS2, as revealed by high‐resolution transmission electron microscopy. Piezoresponse force microscopy studies confirm the out‐of‐plane polar axis of the nanowire films and reveal coercive voltages as low as 0.1 V. Reversing the polarization can induce a conductance switching ratio of >108in bilayer ReS2, over six orders of magnitude higher than that achieved by an untreated polymer gate. This study points to a cost‐effective route to large‐scale processing of high‐performance ferroelectric polymer thin films for flexible energy‐efficient nanoelectronics.

     
    more » « less
  5. Abstract

    The intensity‐scan (I‐scan) technique to study the polarization‐dependent, nonlinear processes in exfoliated bulk ReS2is utilized. The polarization‐dependent reflection and transmission of ReS2, from which the absorption coefficients are extracted using the transfer matrix method, are measured. Absorption coefficients under high laser peak power show a transition from saturable absorption (SA) to reverse saturable absorption when rotating the laser polarization with respect to theb‐axis. It is found that SA and excited‐state absorption (ESA) contribute to the nonlinear optical processes. Both the SA and ESA show strong dependence on the polarization angle, which is attributed to the anisotropic optical transition probability and electronic band structure in ReS2. The anisotropic nonlinear optical properties of ReS2may find applications as saturable absorbers in lasers and optical modulators.

     
    more » « less