PL is a non-contact, non-destructive method of probing the electronic structure of materials. Incident photons are directed onto a sample where they are absorbed (photo-excitation) and the sample will then release photons. Quantum mechanically PL can be described as an excitation to a higher energy state and then a return to a lower energy state accompanied by the emission of a photon. Available energy states and allowed transitions between states are determined by the rules of quantum mechanics. The period between absorption and emission is typically extremely short, in the order of 10 nanoseconds.
PL is often used in the context of semiconductor devices where the energy of the incident photons is above the bandgap energy. PL spectra are useful to determine the bandgap energy, the composition of heterostructures, the impurity levels, the crystal quality, and to investigate recombination mechanisms.
Several variations of photoluminescence exist: in the photoluminescence excitation (PLE) the spontaneous emission from the sample is detected at fixed photon energy and recorded as a function of the pump frequency; time-resolved photoluminescence (TRPL) is where you excite luminescence in a sample with a light pulse and then look at the decay in photoluminescence with respect to time.
Helium to room temperature luminescence characterization of solid samples
Analysis of material properties (charge carrier recombination, electronic transition) and the detection of defects or impurities
Multiple laser sources with optical power at the sample between 1 and 10 mW are focused on the sample using a confocal microscope
Spatial resolution between 0.5 and 2 µm depending on the chosen microscope objective
XY mapping stage range: 100 mm x 100 mm
XY mapping stage resolution: 0.1 µm
From few mm2 to 20 mm diameter flat samples inside the cryostat
From few mm2 to 4” diameter flat samples out of the cryostat
Helium cryostat from 4 K to 500 K
CNR-ISM
Italy
Photoluminescence Spectrometer
Photoluminescence (PL) spectroscopy is a classical characterization of semicondutors and it is sensitive of many body and population effects. It is the simplest spectroscopy of excited states and the interpretation in most cases is straightforward because it is a linear technique. In the CNR-ISM laboratory it is possible to perform both steady state and time dependent PL.
PL spectroscopy involves the excitation of the sample using: 405 nm CW; 400 nm @ 80 MHz (frequency-doubled of mode-locked Ti:Sa oscillator); 250 – 800 nm @ 1 KHz from the optical parameter amplifier.
Laser spot size on the sample: 100 um.
The sample can be mounted in air and in a high vacuum cryostat at 10 – 400 K variable temperature.
The samples can be measured at atmospheric pressure or at a pressure of 10E-5 mbar.
The PL is detected in the wavelength range of 400-1000 nm (CW and 80 MHz) and 250-800 nm (1 KHz).
In SEM a beam is scanned over a sample surface while a signal from secondary or back-scattered electrons is recorded. SEM is used to image an area of the sample with nanometric resolution, and also to measure its composition, crystallographic phase distribution and local texture.
XPS is a surface spectroscopic technique for quantitative measurements of the elemental composition or stoichiometry and the chemical state of the present elements, like their oxidation state and chemical bonds. XPS is highly surface sensitive, giving chemical and binding energy information from the a narrow region close to the surface.
In TEM/Scanning TEM (STEM) high energy electrons incident on ultra-thin samples, allow imaging, diffraction, electron energy loss spectroscopy and chemical analysis of solid materials with a spatial resolution on the order of 1-2 Å. Samples must have a thickness of a few tens of nanometres and are prepared in sample preparation laboratory.
XRD provides non-destructive information on the structural order of a material. At large scattering angles XRD permits to identify different crystal phases and to quantify lattice distances and crystalline volume fractions. At low angles of incidence the surface roughness of a single crystal and the thickness of a deposition layer can be obtained.
SAXS is a non-destructive and versatile method to study the nanoscale structure of any type of material (solid, liquid, aerosols) ranging from new nanocomposites to biological macromolecules. Averaged particle sizes, shapes and distributions, porosity, degree of crystallinity and electron density maps with nanometer precision can be obtained.
