Using Terahertz Spectroscopy for Non-Conducting Materials

Low-frequency IR-active vibrational modes in molecular solids can be accessed easily using terahertz spectroscopy. When examining crystalline materials using terahertz, each material will display a unique spectrum in this range. As each molecular solid has a unique spectrum, there is a certain difficulty that arises when attempting to assign the molecular motions present.

In the terahertz range, vibrationally active modes comprise of a combination of bulk external motions (translations and rotations about axes) as well as certain internal molecular motions. It is not possible to tell what kind of motion is present just by observing the experimental spectrum. A particularly effective technique that helps to comprehend the nature of these low-frequency motions is using computational models.

Terahertz Spectroscopy

Terahertz spectroscopy is a non-destructive, non-ionizing spectroscopic method that can penetrate non-conducting materials. This method exhibits promise in many fields ranging from the pharmaceutical industry to medical imaging to art preservation.

When examining crystalline molecular solids, terahertz spectroscopy offers unique, ‘fingerprint’ spectra for each individual molecular solid studied. Deeper analysis is necessary so as to understand the features that exist in the spectra. Using computational methods are very helpful when trying to comprehend and assign motions in the terahertz region.

It is imperative to consider obtaining the terahertz spectra at low temperature (78 K or lower). Terahertz spectra at low temperature have better resolved, sharper peaks. Furthermore, most of the computational techniques will use 0 K as the default temperature for the analysis. Obtaining experimental spectra as close to 0 K as possible can result in improved assignments from the theoretical spectra.

Computational Methods

There are many computational techniques available that can be used to help in the interpretation of terahertz spectra. Although the different techniques use different paths to acquire information in the terahertz region, they all can result in information that can be used to describe and even animate the motions existing within terahertz spectra. By inputting some standard information about the molecular solid in question (atomic positions, unit cell, etc), the calculations can offer optimized unit cell geometries as well as vibrational mode frequency positions and intensities.

TeraView TeraPulse 4000 Terahertz Spectrometer and Cryostat Options

TeraView’s TeraPulse 4000 system (see Figure 1) has world leading signal-to-noise, bandwidth and modular extension options to meet the user’s challenging research requirements. With an installed based over 120 units across 30 countries, TeraView’s knowledge in terahertz instrumentation and pioneering applications work is unmatched amongst vendors, and this experience and know-how has been applied in the design of the TeraPulse 4000.

Four fiber points are provided to support such external fibers, bespoke probes for in-line or other industrial applications, as well as internal or external sample chamber which accepts TeraView’s broad range of plug-and-play modules.

The system can be configured to match the user’s challenging requirements.

TeraPulse 4000 modular system

Figure 1. TeraPulse 4000 modular system (030-9500)–with integrated sample compartment (030-9504) and reflection imaging module (030-9430).

Key features include:

  • Frequency coverage from 0.06 THz to usually 4.5 THz from a single emitter; extendable to 6 THz, with future upgrades to 7.2 THz planned
  • Short warm-up time; without the need for calibration of an electronic delay line
  • Uninterrupted use of the instrument during data acquisition
  • Outstanding spectral resolution of 1.7 GHz, but normally can accomplish 1 GHz as standard without the need for recalibration of optical delay
  • Single laser system: thus, low jitter between receiver and emitter beams allowing detection of layers as thin as 20 µm
  • Broad range of plug and play modules for easy and cost-efficient extension of the instrument’s capabilities

Multiple fiber ports (optional 030-9501) allow the application of plug and play modules and fiber probes without the two experimental configurations interfering with one another.

TeraView offers a range of cryostats which allow for both cooling and heating of the sample over a variety of temperatures.

030-9425 Cooled Variable Temperature Cell

The variable temperature cell is the perfect accessory to perform terahertz transmission spectroscopy on solid or liquid samples at temperatures in the range of -190 °C to 250 °C. The variable temperature cell comprises of Z-cut quartz windows and a vacuum jacket with a refrigerant dewar/cell holder assembly.

Sample cells are inserted into the heating block part of the dewar/cell holder and the entire system is worked within a vacuum environment maintained by the outer jacket. Using a mixture of refrigerant and cell block heaters any temperature from -190 °C to 250 °C can be accomplished. Usually, liquid nitrogen is used as the coolant medium.

Requires 030-9503 or 030-9504.

Sample holders for solid (030-9423) or liquid samples (030-9424) must be ordered individually.

Variable temperature cooled cell.

Figure 2. Variable temperature cooled cell.

030-9428 Cryostat Module

The TeraPulse 4000 can be supplied with a cryostat specially designed for material physics and condensed matter studies. It enables terahertz spectroscopy to be conducted on super/semi-conducting materials and many other sample types within a variety of temperatures from 4 K to 300 K.

Besides the transmission accessory, a reflectance upgrade (030-9429) is also available. This module has a height modification collar that allows the terahertz waves to reflect onto the bottom port of the cryostat.

Potential experiments can provide insight into:

  • Carrier dynamics
  • Drude modeling of carriers
  • Dielectric function
  • Optical conductivity
  • Magneto-resistance
  • Metal insulator transitions

Cryostat option which can be fitted to the sample compartment.

Figure 3. Cryostat option which can be fitted to the sample compartment.

Requires 030-9503 or 030-9504.

The system can currently be provided with a He-free cryostat from Oxford Instruments.

Specifications:

  • Factory modifications to TeraPulse 4000 to allow use of Oxford Instruments Optistat CF Cryostat
  • Fast track Optistat Dry Bottom Loading 3-300 K with water cooled compressor system
  • FTOPTIDRYBL4W Optistat Dry BL based on GM cooler with water cooled compressor, bottom loading optical cryostat package consisting of: DRYSTDV, DRYSKBL, MERC-ITC-1, DRYTSH, DRYLX20, OPTIDRYBL4W and DRYCC1 OPTIDRYBL4W Optistat Dry BL based on GM cooler with water cooled compressor, bottom loading optical cryostat:
    • Cryostat cool down time from room temperature to 4.2 K approximately 160 minutes
    • Temperature range <4 - 300 K
    • Temperature stability ±0.1 K measured over a 30 minute period
    • Five optical access ports (4 radial and 1 axial) for demountable windows. Fitted with blanks as standards
    • Cooling power: 0.2 W at 4.2 K
    • Optical access f/1 in radial directions

He-free cryostat from Oxford instruments.

Figure 4. He-free cryostat from Oxford instruments.

Requires 030-9504 only.

Experimental

Plenty of work has been done using terahertz spectroscopy to analyze crystalline materials. As stated earlier, each terahertz spectrum is unique to the molecular solid being examined. So as to gain more understanding of the vibrational modes present in the unique terahertz spectra, computational techniques can be utilized. Using a combination of experimental terahertz spectroscopy and theoretical computational techniques can provide very valuable insight into the properties of each unique spectrum acquired.

Once experimental and theoretical terahertz spectra are acquired, the two can be compared so as to assign the molecular motions present. The vibrational features existing in the theoretical spectrum can be aligned to the features present in the experimental spectrum. Furthermore, the theoretical spectrum may be able to shed light into experimental absorptions that contain numerous smaller features that cannot be easily distinguished. For a better understanding, the individual absorptions in the theoretical spectrum can be assessed and animated to display the motion of the molecular solid at that frequency. This can be repeated for each absorption present so as to understand all of the motions that are occurring throughout the terahertz frequency range.

Results and Discussion

Using experimental and theoretical terahertz data, the absorptions observed in acyclic diglycine have been explained in a new publication.1 The experimental data was attained at both room temperature (298 K) and at liquid nitrogen temperature (78 K) (Fig. 5). As mentioned beforehand, upon cooling to 78 K, the features in the terahertz region become a lot sharper. In this specific example, what seems to be a large noisy absorption from around 80 – 100 cm-1 resolves into three distinguishable peaks at the lower temperature (80.9, 86.7 and 95.8 cm-1).

A single molecule of acyclic diglycine

Figure 5. Experimental terahertz spectra of acyclic diglycine at 293 K (red) and 78 K (blue). A single molecule of acyclic diglycine is shown.

The active modes in the experimental spectrum can additionally be described by using theoretical simulations. Here, solid-state density functional theory calculations were used to produce the theoretical terahertz spectrum of acyclic diglycine (Figure 6).

This technique of simulation allows for both the theoretical frequency positions and intensities of the absorptions to be calculated. Once both the experimental and theoretical spectra are present, comparisons can be done and the peaks can be arranged and assigned. Figure 6 illustrates the experimental spectrum on the bottom and the results of the solid-state density functional theory simulation on the top.

Experimental 78 K spectrum of acyclic digylcine

Figure 6. Experimental 78 K spectrum of acyclic digylcine (bottom) and solid-state density functional theory spectrum of acyclic diglycine (bottom).

Besides the ability to assign the peak locations, deeper analysis can reveal the types of motion that happen at each absorption in the spectrum. Using information from the frequency calculations, the vibrational modes can be animated and actually display the motions taking place in the molecular solids. In the case of the acyclic diglycine, the molecular motions include external rotations about the a- and c-axes as well as certain torsions within the molecules themselves.

Conclusions

Latest work has revealed that, along with computational approaches, terahertz spectral features can be described and assigned. Using the information acquired from the theoretical terahertz spectra, experimental absorptions can be assigned and their motions can be described for each individual molecular solid examined. The combination of experimental and theoretical terahertz spectroscopy is a robust tool that can be used to explain the detected features in the low-frequency range.

This information has been sourced, reviewed and adapted from materials provided by TeraView Ltd.

For more information on this source, please visit TeraView Ltd.

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