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Emerging Applications of THz-Raman Analyzers

Overview – Nanostructure Analysis

THz-Raman™ spectroscopy is a relatively new analytical modality that is unique among optical spectroscopic methods in providing unambiguous and quantitative data about the nanoscale structure of samples, in real time and with no special sample preparation required. It is therefore an ideal tool for virtually any R&D, process monitoring or QC/QA application, where the functional performance depends not just on the chemistry of the sample but also on its nanoscale structure. These applications include colloids where particle size can be critical, polymers where crystallinity and layer thickness determine physical (e.g., tensile strength, flowability) properties, liquid crystal display technology where molecular alignment determines light transmission efficiency, quantum dots for displays, solar, and other photonic  applications where the size of the dots determines performance, as well as other 2D materials such as perovskite films where film thickness is a key parameter governing photonic functions. The ability to make these measurements using simple push button analyzers and even in-process through a glass window without special sample preparation or complex instrumentation are major advantages for THz-Raman over non-spectroscopic techniques such as powder X-ray diffraction (PRXD). As described in detail in an earlier whitepaper, Pharma Applications of THz-Raman Quantitative Analysis, THz-Raman has rapidly gained acceptance in the pharmaceutical industry, for example where polymorphic or cocrystal details directly impact dosing levels. In this whitepaper we take a look at some interesting new applications in other industries and technologies.

  • ポリマー*とコポリマー*
  • 半導体ナノ結晶(量子ドット)
  • ペロブスカイト薄膜*
  • 2D素材*
  • 発光性有機結晶(ルブレン)
  • 液晶
  • コロイド
  • 粘着剤の硬化*
  • 不正原料の出所表示
  • 酸素リークモニタリング

 
*本ホワイトペーパーに掲載されているデータ例

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Some THz-Raman Background Information

THz-Raman spectroscopy (also called low-frequency Raman) is a type of vibrational spectroscopy. While traditional techniques like FTIR and conventional Raman detect intramolecular vibrations, THz-Raman detects larger scale vibrations which naturally occur in a much lower frequency domain, roughly 0.15 THz to 6 THz (i.e., 5 cm-1 to 200 cm-1), These vibrations include lattice vibrations in solids, i.e., phonons, deformations of biomolecular entities such as proteins and even viruses, motions of polymer chains, vibrations of large particles like colloids, “breathing” movements of thin layers, and so forth. So, these vibrational modes provide information about the size, shape, conformation and order of samples, including local phase (crystalline vs. amorphous). In addition, the ratio of anti-Stokes to Stokes shifted Raman peaks indicate the populations of these lower lying vibrational states, yielding Boltzmann temperature data about the sample.

A comprehensive discussion of the THz-Raman spectroscopic method is available in an earlier whitepaper, An Introduction to THz-Raman Spectroscopy. Although the study of THz vibrations has long been known to deliver a wealth of unique information, THz-Raman was historically challenging due to the problem of separating the weak Raman-scatter light from the more intense (Rayleigh) scattered light at the original wavelength. Ondax (now Coherent) engineers solved this challenge with a new type of glass filter called a volume Bragg grating or VBG. Together with the company’s laser expertise and vertical integration, this allowed Coherent to offer a comprehensive range of THz-Raman tools. These range from components to turnkey modules that extend existing Raman spectrometers to the THz regime, to microscope modules, benchtop analyzers for conventional sample vials, immersion probes for at-line analysis of powders and liquids, to complete research-grade spectrometers. This product line provides a common and user-friendly platform for the research lab, R&D applications development, process monitoring, and sample QC/QA. 

The following diverse examples are just a few of the numerous emerging applications that can benefit from the unique data now available with these THz-Raman tools.

 

Polymers

High Density Polyethylene’s (HDPE) commercial appeal lies not only in its cost competitiveness but also its wide-ranging versatility. For example, HDPE’s structural properties can be modified via annealing to affect the long-range order over a certain critical distance, known as the lamellar thickness. Varying the annealing method leads to changes in the lamellar thickness and the degree of crystallinity, which can directly affect the mechanical performance of the plastic. This variability makes it useful in applications from milk jugs and bottle caps to plastic bags and plastic lumber.

Traditional spectroscopic methods that target chemical identification are not particularly useful for analyzing these different types of HDPE because they are all the same material. However, THz-Raman is ideal for this type of analysis since it yields data about low-frequency modes that are strongly influenced by phase details such as the degree of crystallinity and lamellar thickness. And because THz-Raman requires no sample preparation and is a non-contact tool, it can be an ideal solution for online or at-line monitoring of changes in polymer structure in real-time.

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図1: ラメラの厚さが異なる4種類のHDPEサンプルの完全なRamanスペクトル。 大きなスペクトルの違いは、THz(<200 cm-1)領域のみに見られます。 出典[1]。

 

A recent study of HDPE samples in the spectroscopy applications laboratory at Coherent confirmed the efficacy of THz-Raman for this application using a Coherent TR-Micro-785 analyzer [1]. Figure 1 shows typical complete THz-Raman spectra from this study of four HPDE samples with different lamellar thickness. As expected, the Raman “chemical fingerprint” spectra (from 200 to 2000 cm-1) for these four samples are remarkably similar. However, there is a clear difference in the peak positions and band shape for the THz region (<200 cm-1) which encompasses the longitudinal acoustic modes.

 

Copolymers

Copolymers are polymers made from more than one type of monomer, where biopolymers are the most common commercial types. These are used in products ranging from automobile tires to stretch fabrics and many common plastic components. Poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate] or PHBHx is a bio-based and completely biodegradable semicrystalline thermoplastic copolymer with various other attractive properties for commercial applications. A recently published study by Noda et al in Applied Spectroscopy [2] combined mid-frequency Raman and THz-Raman measurements to study the isothermal crystallization of PHBHx. By combining measurements of the mid-frequency CO stretching and the THz lattice modes, these researchers were able to perform hetero-mode correlation analysis to deliver 2D spectroscopic data. (Incidentally, these researchers noted in this publication that the high optical throughput and filtering efficiency (i.e., Rayleigh blocking) of the Coherent probe head enabled them to completely dispense with the holographic notch filter normally integrated within their mid-frequency Raman spectrometer.)

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図 2: PHBHxの結晶化に関与する異なる相の特徴を示す非同期2次元ラマン相関スペクトル。 許可を得て[2]より転載。

図2は、本研究で得られた非同期2次元Raman相関スペクトルをマージした例です。 66~96 cm-1ウィンドウの低周波のヘリックス間およびヘリックス内の振動モードに関する既知の分光学的情報と合わせて、スペクトルの特徴をPHBHxの異なる状態 (非晶質、整然とした初晶、二次結晶、完全形成ラメラ)に明確に割り当てることができるようになりました。 これらのスペクトル特徴の変化をリアルタイムで追うことで、結晶化過程の詳細なシーケンスを初めて追跡することができました。 このようなデータを活用することで、共重合体の調製や結晶化を監視・操作し、弾性率などの物性を決定論的に最適化することができます。

Metal Halide Perovskite (MHP) Polymorphs

Perovskite films show great potential for solar, LEDs, photodetectors, and other photonic applications. For example, hybrid lead halide perovskites have already demonstrated solar conversion efficiencies >20%. However, much research is still required to understand their photonic properties in greater detail: in order to ultimately optimize films and devices for efficiency, reliability, and longevity. To this end, a recently published study by Yang et al in Journal of Materials Chemistry C [3] used a Coherent THz-Raman benchtop module to study low frequency vibrational modes in hybrid lead halide perovskites, specifically CsPbIXBr3-X and MAYCs1-YPbI3. This type of information is important because these modes affect the materials’ photonic properties through electron-phonon coupling. Another study of lead-halide perovskites by Guo et al in Nature Communications [4] used low-frequency Raman to help determine the consequence of strong lattice anharmonicity on carrier recombination luminescence.

Figure 3: Low-frequency Raman spectra collected at room temperature. From [3] with permission..

これらの物質のうち、γ-CsPbI3などの興味深い多形は、室温で準安定であり、湿気によって劣化し、バンドギャップ以上のエネルギーのレーザ光にさらされると劣化するものがあります。 (幸いなことに、THz-Ramanの信号強度は高いため、より低いレーザ出力で使用することができます。) そこで研究者らは、安定化テンプレートを用いて、陽極酸化アルミニウム(AAO)テンプレートの直径20~250 nmの円筒形ナノ穴に、溶液中の様々なペロブスカイト材料を蒸着させました。 この外部安定化により、THz-Ramanモジュールを用いた研究の延長が可能になりました。 このモジュールには、試料からの蛍光を避けるために976 nmのレーザ光源が搭載されていますが、標準的なシリコンベースのCCD検出器が使用されています(図3参照)。

この研究では、相転移をリアルタイムで観察するなど、多くの興味深い観察と結論が得られています。 特に、化学的置換により、これら一連の金属ハロゲン化ペロブスカイトの格子振動のエネルギーを支配する主要因は、イオンレーザの性質ではなく、格子サイズであることが示されました。 具体的には、格子間隔の増加とともにピーク位置が低波数側にシフトすることがわかりました。 このことから、フォトニックデバイスの効率に影響を与える電子-フォノン結合の相互作用を操作するためには、格子間隔の考慮が重要な要素になることがわかります。

 

Monitoring Adhesive Curing in Real Time

Ensuring proper curing of epoxies during the manufacturing process is a critical consideration across most manufacturing sectors.  Raman spectroscopy is one method that has been employed to monitor epoxy crossing linking process. Most of these Raman measurements have been limited to the chemical fingerprint region between 500 cm-1 and 2000 cm-1, where the spectral peak variations are actually quite small.  

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図 4: (上)「5分」エポキシ樹脂の25分間に記録された多数のRamanスペクトルのフォールスカラーコンポジット。 (下)このデータから得られたスペクトル分散のプロット。

 

最近、Coherentのアプリケーションラボで、硬化過程をモニターするためのTHz-Ramanの有用性を評価する研究が行われました。 市販のエポキシ樹脂を数種類、硬化時間を5分から120分まで指定して検討しました。 手動で混合した後、Coherent TR-Probe(808 nmレーザ搭載)とステアラブル非接触光学アクセサリーを倒立顕微鏡として使用し、各サンプルのスペクトルを取得しました。 各エポキシタイプについて、混合などのランダムな変動を考慮し、複数のサンプルを実行しました。

図4は、この研究で得られた代表的なデータです。メーカーによると、公称5分のセットタイムと1時間の硬化時間を持つエポキシの場合です。 上図は、25分間に1分間隔で取得したスペクトルをフォールスカラーで表示したものです。 低周波(200 cm-1未満)において、その差が非常に大きいことは、一目見ただけで明らかです。 これらのRamanシグナルは、架橋中のエポキシのバルクせん断モードとブリージングモードの変化と相関していると思われます。 各スペクトル系列を正規化し、スペクトル分散のプロットに変換すると、その差はさらに顕著になります。 その典型的な例を図4(下)に示します。 化学指紋領域(すなわち「従来の」Ramanスペクトル)の強度変化(分散)は、低周波の分散と同様の大きさにするために、2桁のスケールアップが必要であることに注目してください。

数種類のエポキシ樹脂を詳細に分析した結果、20 cm-1と85 cm-1におけるラマン強度の比が、エポキシ硬化プロセスのキネティクスを監視するための有望な指標となることが示されました。 図5は、異なるエポキシ樹脂のサンプルについて、この比率をプロットしたものです。このサンプルは、メーカーによれば、公称120分のセットタイムと24時間の硬化時間を持つ「海洋」タイプの接着剤です。 この研究では、低速硬化と高速硬化の異なるエポキシ樹脂についても同様のデータが得られ、好気性と嫌気性の両方の硬化条件におけるエポキシ樹脂のモニタリングに、低周波Ramanデータが幅広く利用できることがさらに確認されました。非破壊、非接触、高速というTHz-Raman測定の特性により、これらの接着剤やその他の工業用接着剤の完全硬化時間や硬化時間のカスタマイズプロセスを開発し、高品質の接着を確保しながら時間を短縮することが容易になります。

図5: 硬化時間120分の手動混合エポキシ樹脂の20 cm-1および85 cm-1におけるラマン強度の比率のリアルタイムプロット。

Layer Properties in 2D Materials

Single- or few-layered materials, often called 2D materials, have dramatically different properties from their bulk phase. A group of these 2D materials based on transition metal dichalcogenide (TMD) exhibit unique and useful electronic and photonic characteristics. Moreover, these are tunable characteristics that are largely governed by the interactions between the vertically stacked layers, and are affected by the number and orientation of the layers.

Interest in layered 2D systems of a type called van der Waals heterostructures increased even further in 2018 when scientists were able to produce samples of dual graphene layers with a so-called “magic angle” twist of 1.1° between the two layers. As predicted by theory, they found regions of superconductivity that could be turned on and off by just a small voltage.

As applied scientists and device engineers seek to explore these and numerous other examples of interesting 2D materials, a simple tool to analyze and monitor the interaction between the layers can be invaluable. THz-Raman analysis is proving just such a tool. While conventional Raman analysis can reveal the chemical details of the layers, extending Raman into the THz domain provides a direct measuring tool for analyzing inter-layer vibrations. The characteristics of these vibrations then delivers quantitative data on the interaction forces between the layers. THz-Raman is the only analysis modality that provides a simple, non-destructive means of obtaining this data, and requires no sample prep, which can enable direct in situ measurements.

There are two classes of these low-frequency inter-layer vibrations in 2D materials. Vibrations where the layers slide relative to each other with virtually no change in the layer distance are called shear vibrations or shear modes. Whereas vibrations involving movement perpendicular to the layers, i.e., changing the inter-layer distance, are called breathing modes or breathing vibrations. 

 

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図6: 2層構造MoSe2材料の正規化THz-Raman™スペクトル。18cm-1のせん断モードピークに対応するピークのシフトと大きさの変化を、回転した層配向(赤)と同心円(青)で示したものです。 許可を得て[4]より転載。

層数が2層から増えるにつれて、層間振動の周波数は増加し、プラトー値になります。 その結果、低周波の振動データは、層の積層構成だけでなく、層数を特徴づける指紋の役割を果たします。 最近発表されたPuretzkyらの研究(ACS Nano誌[5])は、この方法でTHz-Ramanを用いて、2層および3層のMoSe2およびWSe2結晶の形の2次元金属ダイカルコゲニドを調査したものです。 本研究におけるRamanスペクトルは、Coherent TR-MICRO-532 THz-Ramanシステムに1段分光器(分光分解能1.5 cm-1)を接続して532 nmで励起し、収集されました。 図6に、MoSe2二層膜の2つの配向(回転と同心円)に対するせん断モードの規格化THz-Ramanスペクトルの例を示します。

ピークが18 cm-1(すなわち0.54 THz)に集中していることに注目してください。これは非常に低い周波数であり、他の手段では検出が極めて困難なものです。

Other Applications

There are numerous other emerging applications where the nanoscale structure and local phase of samples directly impacts functions. Some of these include semiconductor quantum dots for LED and other photonic devices, luminescent organic crystals (e.g., rubrene, tetracene) and liquid crystals used in some displays, and colloidal suspensions, to name just a few. There are also some applications where the main purpose is trace analysis rather than functional impact. One pf these is detecting air leaks in oil and gas pipelines, taking advantage of the high sensitivity for THz-Raman to detection oxygen. Another is finding signatures of narcotics such as synthetic cannabinoids to determine the source. Even counterfeit detection and prevention of commercially available materials can be improved with THz-Raman. These applications are possible because the same chemical material can have THz-Raman spectra that help identify and differentiate synthetic pathways, ingredients, and formulations, as well as reveal changes relating to environment and storage (e.g., heat, humidity). Each of these factors can leave behind telltale “signatures” or “fingerprints” in the molecular structure that act like water marks to help the forensic specialist in narrowing or accelerating their search for the source.

 

Summary

THz-Raman is a newly available analytical modality providing the structural and phase data that formerly required cumbersome X-ray diffraction or other complex methods. But it comes with the ease of use of optical spectroscopy, including non-contact, fast analysis, and no special sample preparation. It is uniquely well-suited for any application where function depends on the nanoscale structure, phase, or order/disorder of a material in bulk, trace or 2D material analysis applications.

References

    1. Coherent, Inc., Structural Changes in Polymers, Application Highlights, 2017

    2. I. Noda et al, Two-Dimensional Raman Correlation Spectroscopy Study of Poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate] Copolymers, Applied Spectroscopy. Vol 71, Number 7, 2017

    3. Y. Yang, Probing lattice vibrations of stabilized CsPbI3 polymorphs via low-frequency Raman spectroscopy, J. Mater. Chem. C. Vol 8, 8896, (2020) 

    4.  Y. Guo et al, Dynamic emission Stokes shift and liquid-like dielectric solvation of band edge carriers in lead-halide perovskites, Nat Commun 10, 1175 (2019).

     5. A.A. Puretzky et al, Low-Frequency Raman Fingerprints of Two-Dimensional Metal Dichalcogenide Layer Stacking Configurations, ACS Nano, Vol 9, Number 6, 6333 (2015)

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