Diamond anvil cell (DAC) Fourier-transform infrared (FTIR) spectroscopy is a powerful technique for investigating the behavior of nanomaterials under high pressure. By coupling the extreme pressure capabilities of a DAC with the molecular-level sensitivity of FTIR, researchers can probe pressure-induced structural transformations, bond compression effects, and metastable phase changes in nanoscale systems. This approach provides critical insights into nanomaterial stability, mechanical properties, and potential high-pressure applications.

The diamond anvil cell generates pressures exceeding 300 GPa while maintaining optical transparency, allowing simultaneous FTIR measurements. Nanomaterials exhibit unique pressure responses compared to bulk materials due to their high surface-to-volume ratios and quantum confinement effects. Pressure-induced amorphization is commonly observed when nanomaterials lose long-range order while retaining short-range bonding. For example, C60 fullerenes begin polymerizing at 1.5 GPa, forming dimeric structures visible through the appearance of new IR-active modes between 700-900 cm-1. By 22 GPa, complete amorphization occurs, evidenced by broadening of the pentagonal pinch mode at 1469 cm-1 and disappearance of crystalline vibrational signatures.

Bond compression shifts in nanomaterials under pressure provide direct information about interatomic potential anharmonicity and bulk modulus. Metal-organic frameworks (MOFs) like ZIF-8 show systematic shifts of Zn-N stretching vibrations from 220 cm-1 at ambient pressure to 260 cm-1 at 3 GPa, indicating bond stiffening. The pressure dependence of these modes yields a bulk modulus of 6.7 GPa, significantly lower than conventional ceramics due to the porous framework structure. In carbon nanotubes, the G-band Raman mode softens initially due to tube ovalization, then hardens above 10 GPa as sp2 bonds compress, with pressure coefficients ranging from -4 to +8 cm-1/GPa depending on chirality.

Quenchable phase changes are particularly important for creating novel nanomaterials with ambient-pressure metastable structures. High-pressure DAC-FTIR studies revealed that TiO2 nanoparticles transform from anatase to baddeleyite phase at 15 GPa, with characteristic O-Ti-O bending modes shifting from 320 cm-1 to 380 cm-1. Upon decompression, the high-pressure phase persists below 5 GPa due to kinetic trapping. Similarly, zeolitic imidazolate frameworks undergo irreversible amorphization above 2 GPa, with the process monitored through disappearance of framework vibration modes at 1580 cm-1 (C=N stretch) and 425 cm-1 (Zn-N bend).

Pressure-transmitting media selection critically affects DAC-FTIR measurements on nanomaterials. Hydrostatic media like neon or argon maintain uniform pressure distribution, while non-hydrostatic conditions can induce anisotropic stress effects. For example, silica nanoparticles show different amorphization pathways under hydrostatic versus non-hydrostatic compression, evidenced by changes in the 800 cm-1 Si-O-Si bending mode broadening kinetics. Pressure calibration using ruby fluorescence or diamond Raman edge ensures accurate pressure determination during FTIR measurements.

Gigapascal-scale studies of nanomaterial mechanical properties often reveal size-dependent effects. Gold nanoparticles smaller than 5 nm exhibit surface premelting at pressures 20% lower than bulk gold, detected through damping of the Au-Au stretching mode at 180 cm-1. Metal oxide nanoparticles like ZnO show pressure-induced wurtzite-to-rocksalt transitions at thresholds inversely proportional to particle size, with 5 nm particles transforming at 9 GPa compared to 12 GPa for bulk material. These transitions are identified by the disappearance of the characteristic E1(TO) mode at 410 cm-1 and emergence of new infrared-active phonons above the transition pressure.

The high surface area of nanomaterials leads to pronounced pressure effects on surface terminations and adsorbates. FTIR studies of hydroxylated silica nanoparticles under pressure show hydrogen bonding network reorganization, with Si-OH stretching vibrations at 3740 cm-1 shifting by -15 cm-1/GPa. In functionalized carbon nanotubes, pressure-induced conformational changes of surface groups appear as intensity variations in C-H stretching modes between 2800-3000 cm-1. These surface effects can dominate the high-pressure response at particle sizes below 10 nm.

DAC-FTIR techniques have been extended to study nanomaterial behavior under combined high pressure and temperature. Resistive or laser heating in the cell allows investigation of pressure-temperature phase diagrams. For instance, boron nitride nanotubes show irreversible conversion to cubic BN at 15 GPa and 1500K, monitored through the disappearance of the 1370 cm-1 in-plane B-N vibration and growth of a new TO mode at 1055 cm-1. Time-resolved high-pressure FTIR can capture kinetic processes like nanoparticle coalescence or pressure-driven chemical reactions.

Recent advances in synchrotron-based infrared microscopy have enhanced DAC-FTIR capabilities for nanomaterials. The bright synchrotron source enables mapping pressure gradients and detecting minor phases in heterogeneous samples. This approach revealed pressure-induced demixing in CdSe/ZnS core-shell quantum dots, with distinct FTIR signatures for each component appearing above 8 GPa. Focal plane array detectors allow simultaneous acquisition of multiple spectra, facilitating studies of pressure distribution effects on nanoparticle ensembles.

The mechanical stability of nanomaterials under pressure has important implications for their technological applications. DAC-FTIR studies demonstrated that graphene oxide membranes maintain structural integrity up to 40 GPa, with progressive pressure-induced reduction evidenced by decreasing C=O stretch intensity at 1720 cm-1. These findings guide the design of nanomaterial-based pressure sensors and containment systems. Similarly, pressure-quenched phases of nanoparticles often exhibit enhanced functional properties, such as the high photocatalytic activity of quenched high-pressure TiO2 phases.

Future developments in DAC-FTIR methodology will focus on improving spatial resolution for single-nanoparticle studies and extending the accessible pressure range into the terapascal regime. Combined with other in-situ probes like X-ray diffraction and optical spectroscopy, these advances will provide a more complete understanding of nanomaterial behavior under extreme conditions. The fundamental insights gained from high-pressure FTIR studies continue to inform both theoretical models of nanoscale materials and practical applications across materials science, energy storage, and nanotechnology.