Cryogenics enhances spectroscopy by improving the SNR, enabling high-resolution measurements, stabilizing samples, enhancing fluorescence and luminescence detection, and reducing background interference.
The Los Angelos Lakers played the Boston Celtics in the Boston Garden in basketball’s biggest rivalry. A solitary Lakers supporter sat among the sea of Boston fans, yelling and cheering loudly. But he was drowned out by the noise of the countless Boston fans. There is no way to identify the Laker fan.
Filtering that noise is analogous to using cryogenics in several analytical chemical applications. Low-temperature techniques help quiet some competing energy levels from other molecules so the ones of interest can be studied.
Temperature control is a technique for selectively probing specific material energy states. We use cryogenics to achieve low temperatures in the broadest possible sense. There are some aspects ofmaterials that sometimes we may be interested in, but the noise around it crowds out the signature of that substance of interest. “Noise” means that competing transitions in other energy states of the material prevent us from seeing what we want to detect for a particular state.
At room temperature, the other molecules are moving around in different energy states, in constant motion. What low temperature does to a material is quiet down the surrounding materials by taking the energy out.
Certain applications require samples at low temperatures. Cryogenics is customarily used in three areas:
Research in quantum computing applications is experiencing significant growth, alongside advancements in semiconductor materials for various applications such as power electronics, electric vehicles (EVs), LEDs, and photovoltaics (PV). This surge is driven by increasing demand from both businesses and consumers for enhanced power electronics, EVs, LEDs, and photovoltaic solutions.
One of the core needs in the semiconductor industry is yield improvement. This is what is responsible for the continuous lowering of cost and improvement in performance. To achieve the goal of high yields, it is essential to reduce or eliminate the defects that occur in the wafers that are used for these devices. Some of these defect states can only be characterized at low temperatures - hence cryogenics.
Condensed matter physics is just the general idea to understand the band structure of the material. With Quantum Physics, it’s getting the molecules into a well-defined quantum state.
In these cases, we achieve that low temperature by using cryogenic systems. The cryogen is just a refrigeration technique. We use cryogenic materials to achieve the low-temperature.
Cryogenics deals with extremely low temperatures, typically below -150 degrees Celsius.
There are usually two cryogenic fluids that people use; helium and liquid nitrogen. Helium goes down to the lowest temperature, which is 4K (Kelvin). And even the quantum computing systems can go to even microkelvin, close to absolute zero.
The Cryostat is just a sample handling and conditioning system, and this is usually incorporated into the analytical measurement system such as the multimodal microspectroscopy system (SMS) from HORIBA. The SMS can enable the multimodal performance of several micro-spectroscopies at temperatures down to 4K (Kelvin). These include Raman, photoluminescence, time-resolved photoluminescence, reflectance, electroluminescence, photocurrent, and darkfield scattering.
Cryogenic solutions generally have been in the domain of do-it-yourself system engineering with add-on microspectroscopy systems rather than turnkey units. This is particularly true in multimodal systems, where multiple spectroscopies are used on the sample with a single instrument. However, some manufacturers provide turnkey, multimodal cryogenic microspectroscopy systems that incorporate multimodal capabilities along with the cryogenic capacity, without moving the cryostat from one modality to another.
Cryogenics enhances the capabilities of various spectroscopic techniques by improving resolution, sensitivity, and stability, enabling the study of unique low-temperature phenomena, and facilitating the analysis of otherwise challenging samples. In general, cryogenics plays a crucial role in enhancing spectroscopy analysis in the following ways:
Reduction of Thermal Noise: Cooling detectors to cryogenic temperatures significantly reduces thermal noise, which is the random motion of electrons caused by heat. This leads to a clearer signal and better SNR. Enhanced Sensitivity: Lower temperatures can improve the sensitivity of detectors, allowing for the detection of weaker signals that might otherwise be lost in the noise at higher temperatures.
Reduced Line Broadening: At higher temperatures, molecular vibrations, and rotations can cause spectral lines to broaden. Cooling the sample can reduce these thermal motions, leading to sharper, more defined spectral lines.
Enhanced Resolution: Cryogenic temperatures help achieve higher resolution in spectroscopy by minimizing Doppler broadening, which is the broadening of spectral lines due to the motion of atoms or molecules.
Preservation of Samples: Some samples are volatile or degrade at room temperature. Cooling them to cryogenic temperatures can stabilize these samples, preventing degradation or reactions that could alter the spectral data.
Study of Low-Temperature Phenomena: Certain materials or biological samples exhibit unique properties at low temperatures that can be studied using cryogenic spectroscopy, providing insights into phenomena that are not observable at higher temperatures.
Increased Fluorescence Efficiency: Cooling can reduce non-radiative decay processes, enhancing the fluorescence yield of a sample. This is particularly useful in fluorescence spectroscopy.
Better Luminescence Detection: Lower temperatures can lead to more efficient phosphorescence and delayed fluorescence, which are beneficial for luminescence spectroscopy.
Minimized Background Radiation: Cryogenic environments can help reduce background infrared radiation, which can interfere with the detection of weak signals, especially in infrared spectroscopy.
Suppression of Unwanted Emissions: Cooling can suppress unwanted emissions from the sample or the surroundings, leading to cleaner spectra.
Raman Spectroscopy: Cryogenic cooling can enhance Raman signals by reducing thermal broadening and improving the detection of weak Raman shifts.
Infrared (IR) Spectroscopy: Cooling infrared detectors improves their sensitivity and reduces noise, which is crucial for detecting weak infrared signals.
X-ray Spectroscopy: Cryogenic cooling of X-ray detectors, such as superconducting transition-edge sensors, improves energy resolution and detection sensitivity.
Nuclear Magnetic Resonance (NMR) Spectroscopy: Cryogenically cooled superconducting magnets produce stronger and more stable magnetic fields, improving spectral resolution. Cryogenics enhances spectroscopy by improving the SNR, enabling high-resolution measurements, stabilizing samples, enhancing fluorescence and luminescence detection, and reducing background interference. These benefits make cryogenics an essential tool in various spectroscopic analyses.
Using cryogenics in conjunction with Raman spectroscopy offers several significant advantages:
Reduction of Thermal Noise: Lowering the temperature reduces thermal vibrations of the sample’s molecules, which can minimize background noise and result in clearer and more precise Raman spectra. This is particularly beneficial for detecting weak Raman signals.
Improved Resolution: Cooling can sharpen the spectral lines because it reduces the Doppler broadening caused by molecular motion. This allows for better resolution and more accurate identification of closely spaced spectral features.
Enhanced Raman Signal: Some Raman- active4modes may have enhanced intensity at low temperatures, making detecting and analyzing specific vibrational modes that might be weak or obscured at higher temperatures easier.
Phase Stability: Certain materials or phases are only stable at low temperatures. Cryogenic conditions can preserve these phases, allowing for the study of materials and their properties that are not stable at room temperature.
Minimization of Photodegradation: For some samples, especially organic and biological specimens, high-intensity laser light used in Raman spectroscopy can cause photodegradation. Cooling the sample can help mitigate this effect, preserving the integrity of the sample during analysis.
Observation of Low-Energy Excitations: Cryogenic temperatures can allow for the observation of low-energy excitations and phonons that might be thermally activated and thus masked at higher temperatures.
Investigation of Quantum Effects: At cryogenic temperatures, quantum mechanical effects become more pronounced. This is particularly useful for studying the quantum behavior of materials and the interactions at a fundamental level.
The use of cryogenics in Raman spectroscopy enhances the quality and range of the data that can be obtained, making it a powerful technique for advanced material characterization, especially in fields like condensed matter physics, chemistry, and materials science.
In a low energy state under certain conditions, these electrons can absorb light and then transition to a high energy state. Of course, nature doesn’t like things to be in this equilibrium. So after a while, that material will fall back down to normal levels. And in doing so, it has to give up that energy through some mechanism, either as heat or light. And when it does give over light, we see things like photoluminescence. That light has a certain wavelength. So we can use that wavelength to understand the energy structure for this material.
In the context of semiconductors, we are interested in things like defects. Those defects introduce foreign energy states in the material, called defect states. Defects or anything foreign in the material or introduces its own energy level, different from the surrounding energy levels.
What happens with cryogenics and low-temperature work is that we get down the energy in the system. Most of the material drops down to that state. So you don’t have all these ambient photons going, for example. And then that gives you a chance to specifically target just that defect state and look at its spectral signature.
The defect can be at a higher or lower energy level. But the point is, by quieting everything around it, you can focus just on the defect.
In quantum science, the idea is just to understand the band structure of materials, because that affects the material properties. And the same with condensed matter physics.
All these applications benefit from cryogenics. While traditionally a site-engineered home-built system, multimodal microspectroscopy systems incorporating cryogenic capabilities exist and can be acquired modularly, so these can be adapted to future needs.
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