Temperature-Dependent Photoluminescence (TD-PL)

Introduction to Temperature-Dependent PL:

Temperature-dependent photoluminescence (TD-PL) is a valuable technique in materials science and condensed matter physics, providing insights into the electronic and optical properties of materials as a function of temperature. In TD-PL studies, the photoluminescence spectra of a material are measured at different temperatures, allowing researchers to observe changes in emission characteristics, energy band structures, and carrier dynamics.

How to Measure Temperature-Dependent PL:

Measuring Temperature-Dependent Photoluminescence (TD-PL) involves utilizing a photoluminescence spectrometer equipped with a temperature control system. The sample, typically mounted in a cryostat or variable-temperature chamber, undergoes a gradual temperature change. Photoluminescence spectra are then recorded at each temperature using a suitable detector. This process allows researchers to observe changes in emission characteristics, energy band structures, and carrier dynamics as the temperature varies, providing valuable insights into the material's behavior under different thermal conditions.

Applications of Temperature-Dependent PL:

  1. Bandgap Engineering: TD-PL is used to investigate changes in the bandgap of semiconductors and nanomaterials with temperature variations, offering insights into their electronic structures.

  2. Defect Characterization: By analyzing temperature-induced changes in PL, researchers can identify and characterize defects in materials, which is crucial for optimizing their performance in electronic devices.

  3. Quantum Dots and Nanomaterials: TD-PL is widely employed in the study of quantum dots and nanomaterials to understand size-dependent effects, surface states, and quantum confinement at different temperatures.

  4. Semiconductor Device Research: In semiconductor device research, TD-PL helps evaluate the impact of temperature on carrier dynamics, recombination rates, and the overall efficiency of electronic devices.

  5. Material Quality Assessment: Researchers use TD-PL to assess the quality of materials by examining temperature-dependent changes in luminescent properties, aiding in material selection for various applications.

  6. Phosphors and LEDs: In the field of lighting, TD-PL is applied to phosphors and light-emitting diodes (LEDs) to optimize their performance and understand thermal effects on luminescent materials.

In summary, TD-PL is a powerful technique for investigating the temperature-dependent behavior of materials, providing crucial information for the design and optimization of various electronic and optoelectronic devices.

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Photodetectors

 

Temperature-dependent photoluminescence (PL) was used to explore GaTe materials' optical properties on a GaAs(001) substrate at 525 °C (Figure 1). The 10K-PL spectrum revealed two emission bands at 1.465 eV and 1.765 eV, attributed to h-GaTe and m-GaTe phases. The m-GaTe band exhibited six peaks corresponding to free exciton (FXn=1) and its second excited states (FXn=2), with a calculated band gap of 1.821 eV and an exciton binding energy (EX) of 20 meV at 10K. The absence of donor−acceptor pair recombination transitions emphasized dominant features at 1.765 and 1.743 eV associated with acceptor-bound exciton recombination (AX1 and AX2), originating from Ga-vacancies. The broadband from 1.8 to 1.6 eV contained bound excitonic emissions (BX) and corresponding free-to-bound recombination (FB). The lack of the donor−acceptor pair recombination (DAP) transition confirmed the natural p-type behavior of 2D GaTe materials. Acceptor levels EA1 and EA2 were found at 103 and 155 meV above the valence band. The 1.465 eV emission was attributed to the near-band-edge emission of the GaAs substrate. The suppression of m-GaTe band emission on the sapphire substrate led to the conclusion of h-GaTe phase band-edge emission at 1.46 eV, indicating a shallow direct band gap.

 

Reference:
S. H. Huynh, N. Q. Diep, T. V. Le, S. K. Wu, C. W. Liu, D. L. Nguyen, H. C. Wen, W. C. Chou, V. Q. Le, and T. T. Vu. Molecular Beam Epitaxy of Two-Dimensional GaTe Nanostructures on GaAs(001) Substrates: Implication for Near-Infrared  Photodetection. ACS Appl. Nano Mater. 2021, 4, 9, 8913–8921

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Optoelectronic Devices

 

To investigate the influence of temperature on the electronic structure and luminescent characteristics of FAPbBr3 perovskite nanocrystals (PNCs), they examined temperature-dependent photoluminescence (PL) spectra across the range of 80–400 K, as illustrated in Fig. 2(a) and (b). As the temperature increases, a distinct blue shift in the peak wavelength of the PL spectra is observed, accompanied by an augmentation in line width and a reduction in intensity. With higher temperatures, the photon energy of the PL peak becomes larger, and the calculated band gap of FAPbBr3 is about 2.15 eV at 80 K, which is in good agreement with the experimental results. In Fig. 2(c), the green points represent the shift in emission peaks at various temperatures, attributed to the simultaneous effects of lattice expansion and exciton–phonon couplings. The linear fitting of the temperature-dependent evolution of PL peaks for FAPbBr3 PNCs indicates slight changes in PL peaks at temperatures below 175 K, followed by a rapid and pronounced blue shift up to 400 K. This temperature-dependent behavior is associated with the phase transition from the orthorhombic to tetragonal and cubic phases. In Fig. 2(d), the experimental data for PL intensity and full width at half maximum (FWHM) of FAPbBr3 PNCs are presented. The normalized PL and FWHM are plotted against 1/K, revealing both increasing and decreasing tendencies marked by red and green lines, respectively.

 

Reference:

Xiaozhe Wang, Qi Wang, Zhijun Chai, and Wenzhi Wu. The thermal stability of FAPbBr3 nanocrystals from temperature-dependent photoluminescence and first-principles calculations. RSC Adv., 2020, 10, 44373-44381.
https://pubs.rsc.org/en/content/articlehtml/2020/ra/d0ra07668f 

 

 

 

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2D Semiconductor

 

The experiment investigated the temperature-dependent photoluminescence (PL) spectra of monolayer MoS2 (Fig. 3a) over a range from 4 K to 300 K, with 10 K intervals. A and B excitons are typical at room temperature, but only A excitons were observed in this study. Below 150 K, a three-state model of MoS2 reveals the emergence of bound excitons (XL). Fig. 3b depicts the three-state model, involving a ground state, trapped state, and high-energy state. At higher temperatures, carriers at the trapped state lead to A exciton emission through thermal energy. Decreasing temperature weakens thermal effects, resulting in direct transitions to the ground state and XL exciton emission. Both A and XL exciton PL intensities increase as temperature decreases due to reduced nonradiative processes. The study highlights temperature's impact on MoS2 photoluminescence, emphasizing XL exciton emergence and A exciton variation based on the three-state model. Temperature-dependent PL peak energy band for A and XL excitons (Fig. 3c-d) was fitted using the Varshni Equation. A red shift in both excitons' central wavelengths with increasing temperature suggests a diminishing energy bandgap. XL excitons show greater temperature sensitivity (0.1 eV shift in 4–150 K) than A excitons (0.06 eV shift in 4–300 K), indicating stringent conditions for XL exciton generation, observed below 150 K. Deviations between experimental and fitted A exciton data, especially at lower temperatures, may be attributed to lower-energy particle stability and potential overlap with XL excitons in the 0–150 K range.

 

Reference:

Li H., Zhang, X.H. Temperature-dependent photoluminescence and time-resolved photoluminescence study of monolayer molybdenum disulfide. Optical Materials 2020, 107, 110150. doi:10.1016/j.optmat.2020.110150 

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Thin film studies

 

In this work, they investigated the effect of Li doping and temperature on the photoluminescence spectra of ZnO thin film prepared by sol-gel spin-coating technique. The photoluminescence (PL) spectra evolution of undoped and Li-doped ZnO thin films, with temperatures ranging from 12 to 300 K, is depicted in Fig. 4a–d. The spectra consist of two Gaussian bands: the intense green luminescence (GL) band around 2.0 eV attributed to structural defects and the asymmetric blue band (NBE) peaking at approximately 3.27 eV related to exciton recombination. The undoped film exhibits a new emission at 3.02 eV below 175 K, suggesting a shallow donor level, possibly originating from Zni defects. In Li-doped films (2%, 4%, and 5%), the NBE peak remains consistent, signifying film crystallinity enhancement. However, in the 10% Li-doped film, the NBE peak disappears, indicating structural degradation due to increased disorder from high Li incorporation. This film exhibits the lowest XRD peak intensity and smaller crystallite size, suggesting Zn atom reoccupation of interstitial positions, leading to Zni interstitial defect concentration increase and the emergence of a peak at 3.02 eV.

 

Reference:

Aida, M.S., Hjiri, M. Temperature-dependent photoluminescence of Li-doped ZnO. J Mater Sci: Mater Electron 31, 10521–10530 (2020). https://doi.org/10.1007/s10854-020-03600-7

 

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