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Resolving transitions in the mesoscale domain configuration in VO2 using laser speckle pattern analysis.

Seal K, Sharoni A, Messman JM, Lokitz BS, Shaw RW, Schuller IK, Snijders PC, Ward TZ - Sci Rep (2014)

Bottom Line: The configuration and evolution of coexisting mesoscopic domains with contrasting material properties are critical in creating novel functionality through emergent physical properties.However, current approaches that map the domain structure involve either spatially resolved but protracted scanning probe experiments without real time information on the domain evolution, or time resolved spectroscopic experiments lacking domain-scale spatial resolution.Our straightforward analysis of laser speckle patterns across the first order phase transition of VO2 can be generalized to other systems with large scale phase separation and has potential as a powerful method with both spatial and temporal resolution to study phase separation in complex materials.

View Article: PubMed Central - PubMed

Affiliation: 1] Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA [2] Department of Physics &Astronomy, University of Tennessee, Knoxville, TN 37996, USA.

ABSTRACT
The configuration and evolution of coexisting mesoscopic domains with contrasting material properties are critical in creating novel functionality through emergent physical properties. However, current approaches that map the domain structure involve either spatially resolved but protracted scanning probe experiments without real time information on the domain evolution, or time resolved spectroscopic experiments lacking domain-scale spatial resolution. We demonstrate an elegant experimental technique that bridges these local and global methods, giving access to mesoscale information on domain formation and evolution at time scales orders of magnitude faster than current spatially resolved approaches. Our straightforward analysis of laser speckle patterns across the first order phase transition of VO2 can be generalized to other systems with large scale phase separation and has potential as a powerful method with both spatial and temporal resolution to study phase separation in complex materials.

No MeSH data available.


Related in: MedlinePlus

Ellipsometry data above, at, below the TMIT for VO2.The (a) real and (b) imaginary parts show a variation in the dielectric permittivity with wavelength at different temperatures. The (c) real and (d) imaginary parts of the dielectric permittivity and their variation with temperature at the illumination wavelengths used in the speckle measurements show dual-phase behavior across the transition temperature.
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f1: Ellipsometry data above, at, below the TMIT for VO2.The (a) real and (b) imaginary parts show a variation in the dielectric permittivity with wavelength at different temperatures. The (c) real and (d) imaginary parts of the dielectric permittivity and their variation with temperature at the illumination wavelengths used in the speckle measurements show dual-phase behavior across the transition temperature.

Mentions: First we performed spectroscopic ellipsometry measurements as a function of temperature to optically characterize the metal-insulator transition (MIT) of the sample. The ellipsometry data were fitted with a Lorentz oscillator model to obtain ε1(λ) and ε2(λ) as shown in Fig. 1(a–b). From these data we obtained ε(T) at each wavelength, shown in Fig. 1(c–d). Here, a regression analysis was used to simultaneously analyze data taken at three wavelengths and the mean square error (MSE) was used to quantify the difference between the experimental data and model. The unknown parameters were allowed to vary until the minimum MSE was reached. It is clear that the MIT starts close to 40°C. The actual sample temperature lags behind the recorded temperature in the ellipsometry data since the thermocouple was placed on the heating stage and not directly on the sample. The changes in ε1(T) and ε2(T) for the three laser wavelengths reveal that the dielectric constant transitions between two dominant values starting at 40°C and completing at approximately 55°C. At 800 nm, the value of ε1 decreases strongly at the transition but remains above zero, indicating weak metallic behavior. This is expected since this illumination is still above the VO2 plasmon frequency7. This implies that for all our subsequent speckle measurements (see below), the surface scattering is not affected by plasmon absorption. At around 1000 nm, ε1 becomes negative above the MIT, demonstrating the anticipated metallic behavior of VO2 at high temperatures. For wavelengths between 540 and 650 nm, ε1 actually increases with temperature across the MIT. The value of ε2 consistently drops across the MIT at all wavelengths. At the temperature where the fraction of metal domains equals that of insulating domains, i.e. near the MIT15, we obtain the effective dielectric constant using the Maxwell Garnet effective medium approximation: 488 nm: 0.2 + 3i, 633 nm: 0.7 + 3.6i, 800 nm: 0.02 + 2.2i.


Resolving transitions in the mesoscale domain configuration in VO2 using laser speckle pattern analysis.

Seal K, Sharoni A, Messman JM, Lokitz BS, Shaw RW, Schuller IK, Snijders PC, Ward TZ - Sci Rep (2014)

Ellipsometry data above, at, below the TMIT for VO2.The (a) real and (b) imaginary parts show a variation in the dielectric permittivity with wavelength at different temperatures. The (c) real and (d) imaginary parts of the dielectric permittivity and their variation with temperature at the illumination wavelengths used in the speckle measurements show dual-phase behavior across the transition temperature.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4151099&req=5

f1: Ellipsometry data above, at, below the TMIT for VO2.The (a) real and (b) imaginary parts show a variation in the dielectric permittivity with wavelength at different temperatures. The (c) real and (d) imaginary parts of the dielectric permittivity and their variation with temperature at the illumination wavelengths used in the speckle measurements show dual-phase behavior across the transition temperature.
Mentions: First we performed spectroscopic ellipsometry measurements as a function of temperature to optically characterize the metal-insulator transition (MIT) of the sample. The ellipsometry data were fitted with a Lorentz oscillator model to obtain ε1(λ) and ε2(λ) as shown in Fig. 1(a–b). From these data we obtained ε(T) at each wavelength, shown in Fig. 1(c–d). Here, a regression analysis was used to simultaneously analyze data taken at three wavelengths and the mean square error (MSE) was used to quantify the difference between the experimental data and model. The unknown parameters were allowed to vary until the minimum MSE was reached. It is clear that the MIT starts close to 40°C. The actual sample temperature lags behind the recorded temperature in the ellipsometry data since the thermocouple was placed on the heating stage and not directly on the sample. The changes in ε1(T) and ε2(T) for the three laser wavelengths reveal that the dielectric constant transitions between two dominant values starting at 40°C and completing at approximately 55°C. At 800 nm, the value of ε1 decreases strongly at the transition but remains above zero, indicating weak metallic behavior. This is expected since this illumination is still above the VO2 plasmon frequency7. This implies that for all our subsequent speckle measurements (see below), the surface scattering is not affected by plasmon absorption. At around 1000 nm, ε1 becomes negative above the MIT, demonstrating the anticipated metallic behavior of VO2 at high temperatures. For wavelengths between 540 and 650 nm, ε1 actually increases with temperature across the MIT. The value of ε2 consistently drops across the MIT at all wavelengths. At the temperature where the fraction of metal domains equals that of insulating domains, i.e. near the MIT15, we obtain the effective dielectric constant using the Maxwell Garnet effective medium approximation: 488 nm: 0.2 + 3i, 633 nm: 0.7 + 3.6i, 800 nm: 0.02 + 2.2i.

Bottom Line: The configuration and evolution of coexisting mesoscopic domains with contrasting material properties are critical in creating novel functionality through emergent physical properties.However, current approaches that map the domain structure involve either spatially resolved but protracted scanning probe experiments without real time information on the domain evolution, or time resolved spectroscopic experiments lacking domain-scale spatial resolution.Our straightforward analysis of laser speckle patterns across the first order phase transition of VO2 can be generalized to other systems with large scale phase separation and has potential as a powerful method with both spatial and temporal resolution to study phase separation in complex materials.

View Article: PubMed Central - PubMed

Affiliation: 1] Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA [2] Department of Physics &Astronomy, University of Tennessee, Knoxville, TN 37996, USA.

ABSTRACT
The configuration and evolution of coexisting mesoscopic domains with contrasting material properties are critical in creating novel functionality through emergent physical properties. However, current approaches that map the domain structure involve either spatially resolved but protracted scanning probe experiments without real time information on the domain evolution, or time resolved spectroscopic experiments lacking domain-scale spatial resolution. We demonstrate an elegant experimental technique that bridges these local and global methods, giving access to mesoscale information on domain formation and evolution at time scales orders of magnitude faster than current spatially resolved approaches. Our straightforward analysis of laser speckle patterns across the first order phase transition of VO2 can be generalized to other systems with large scale phase separation and has potential as a powerful method with both spatial and temporal resolution to study phase separation in complex materials.

No MeSH data available.


Related in: MedlinePlus