Isotope Doping Research Articles

Growth, first-principles calculations and optical properties of a novel near-infrared Ho: NLGW laser crystal

A rigid structure was constructed using an isotopic doping strategy, and a novel near-infrared 1 at% Ho3+: NaLa0.93Gd0.06(WO4)2 (Ho: NLGW) laser crystal with a size of Φ 18 mm × 30 mm was successfully grown by the Czochralski method. X-ray rocking curves and Rietveld refinement indicate that the grown crystal possesses high crystalline quality and crystallizes in the I41/a space group with lattice parameters: a = b = 5.3518 Å, c = 11.6438 Å and V = 333.5051 Å3. The Ho: NLGW crystal exhibits a low coefficient of thermal expansion (8.472 × 10−6 K−1), and the c-axis thermal conductivity increases from 1.235 W/(m × K) to 1.404 W/(m × K). The thermal properties of the Ho: NLGW crystal surpass those of other Ho3+-doped tungstate series and mainstream near-infrared laser crystals, suggesting that it is easier to obtain excellent laser gain. For revealing the excellent thermal properties, the elastic moduli of the NLW and NLGW crystals have been calculated by first principles. With the introduction of Gd3+, the structural stiffness of the NLW as a whole has been increased to optimize the mechanical properties in each axial direction and to obtain the reason for the larger Debye temperature. Finally, the transmission, absorption and near-infrared emission spectra of the Ho: NLGW crystal were investigated. The crystal exhibits a high transmittance of 88.5 % and an emission cross-section of 0.84 × 10−20 cm2 calculated according to the J-O theory. Experimental results indicate that the grown Ho: NLGW crystal can achieve large laser output more easily and are expected to be an excellent medium for 2 μm solid-state lasers.

A red-emitting Sm3+-activated BaLaMgNbO6 phosphor for WLED applications obtained by using an isotopic doping strategy (Y → La)

BaLa1-x-yMgNbO6: xSm3+, yY3+ phosphors were successfully synthesized via high-temperature solid-state reaction method. The crystal structure of representative samples was determined using X-ray diffraction and Rietveld refinement, confirming high phase purity of the prepared phosphors. In the singly doped BaLa1-xMgNbO6: 0.05 S m3+ sample, red emission was observed under 406 nm excitation with an internal quantum efficiency of 27.8 %. The thermal stability at 599 nm (483 K) retained 72.39 % of the emission intensity. To further enhance the luminescent properties, co-doping strategy was employed by substituting Y3+ for La3+ ions. This adjustment significantly improved the brightness of the phosphors. For instance, in BaLa0.93MgNbO6: 0.05 S m3+, 0.02Y3+, the emission intensity at 599 nm was enhanced by 3.27 times compared to BaLa0.95MgNbO6: 0.05 S m3+, with an internal quantum efficiency increased from 27.8 % to 60.2 %. Moreover, the color purity of BaLa1-x-yMgNbO6: xSm3+, yY3+ phosphors reached 99 %. Warm white LED devices fabricated using BaLa0.93MgNbO6: 0.05 S m3+, 0.02Y3+ exhibited a color rendering index (Ra) of 95.7 and a color temperature of 4938K, indicating broad application potential. Experimental results demonstrate that BaLa0.93MgNbO6: 0.05 S m3+, 0.02Y3+ phosphors with double perovskite structure are a novel and high-performance material for warm white LED phosphors.

Isotope interface engineering for thermal transport suppression in cryogenic graphene

The development of emerging technologies, such as quantum computing and semiconductor electronics, emphasizes the growing significance of thermal management at cryogenic temperatures. Herein, by designing isotope interfaces based on the Golomb ruler, we achieved effective suppression of the phonon thermal transport of cryogenic graphene. The pronounced disordering of the Golomb ruler sequence results in the stronger suppression of thermal transport compared to other sequences with the same isotope doping ratio. We demonstrated that the Golomb ruler-based isotope interfaces have strong scattering and confinement effects on phonon transport via extensive molecular dynamics simulations combined with wave packet analysis, with a proper correction for the missing quantum statistics. This work provides a new stream for the design of thermal transport suppression under cryogenic conditions and is expected to expand to other fields.

Isotope doping-induced crossover shift in the thermal conductivity of thin silicon nanowires

Here, using homogeneous nonequilibrium molecular dynamics simulations, we report the thermal transport characteristics of thin Si nanowires (NWs) with varying size and isotope doping ratio. It is identified that crossover in the thermal conductivity (κ) of both isotope doping-free and isotope doped Si-NWs appears at critical sizes, below which κ is enlarged with decreasing size because the hydrodynamic phonon flow predominates, above which, due to the dominant phonon boundary scattering, opposite behavior is observed. With increasing isotope doping, however, the critical size in minimizing the κ is moved to small values because the phonon impurity scattering caused by isotope doping is critically involved. Moreover, there is a critical isotope doping (<50%) in the critical size motion, originating from that, above which, the critical size no longer moves due to the persistence of hydrodynamic phonon flow. This study provides new insights into the thermal transport behaviors of quasi-1D structures.

Probing the application of kinetic theory to Mg-phyllosilicate growth with Si isotope doping

The Principle of Detailed Balance (PDB) played a defining role in the derivation of the widely-used Transition State Theory rate law equation and serves as an important link between geochemical kinetics and thermodynamics. Although significant improvements have been made in applying the PDB to comparatively simple systems (e.g., SiO2-water), experimental verification of the PDB is lacking for more complex minerals such as the phyllosilicates. Providing kinetic constraints on the rates of phyllosilicate growth from supersaturated solutions is particularly important for constructing facies models, constraining element cycling in the lacustrine and marine environments, and interpreting paleo-biogeochemistry. Here, we use 29Si isotopic doping techniques to quantify the rates of reaction between Mg-phyllosilicate substrates (amorphous Mg-silicate (a talc-like phase) and crystalline talc) and supersaturated solutions at 21 and 60 °C. The results show that the ratio of the forward and backward rates of amorphous Mg-silicate-water reaction approaches unity as the saturation state of the solution approaches the apparent solubility of amorphous Mg-silicate. The precipitation rates coupled with equivalent dissolution rates obtained from experiments with amorphous Mg-silicate substrates appear to obey the same rate function as the precipitation rates coupled with negligible dissolution rates obtained from experiments with crystalline Mg-silicate substrates over the degrees of supersaturation we explore, suggesting that the elementary step limiting the rate of precipitation remains the same. Accordingly, our results demonstrate that the PDB is applicable to amorphous Mg-phyllosilicate-water reactions, thereby reinforcing the use of TST rate equations to describe Mg-phyllosilicate growth. The experimental data can also be taken as evidence that the apparent solubility of amorphous Mg-silicate, a concept previously explained using the kinetic theory of nucleation and growth, also has a thermodynamic meaning, in that it represents a metastable equilibrium with the poorly crystalline phase. The measured, non-negligible forward and backward rates suggest that, even in this metastable state where little if any net reaction is occurring, isotopic signatures can be reset. Moreover, the significant discrepancy between the heterogeneous net growth rates on the amorphous Mg-silicate substrate versus those measured on crystalline talc and sepiolite substrates indicates that mineral crystallinity likely plays a key role in mineral growth during diagenesis.

Phonon localization in single wall carbon nanotube: Combined effect of 13C isotope and vacancies

The combined effect of 13C isotope doping and vacancies on the phonon properties of a single-wall carbon nanotube is theoretically investigated using the forced oscillation method. The phonon density of states (PDOS) is calculated for all (0%–100%) 13C isotope contents and wide (0%–30%) vacancy concentrations. We found a redshift of the Raman active E2g peak in the PDOS with increasing isotope contents, while the disappearance of the E2g peak and the appearance of a new sharp peak in the low-energy region with increasing combined defects. Both 13C isotope and combined defects cause the localization of the high-energy optical phonons. We calculated the typical mode patterns for the in-plane longitudinal optical phonon to visualize the localization phenomena elaborately at the presence of isotope and vacancies. The calculated average localization length shows an asymmetric behavior with increasing 13C isotope concentrations which is in good agreement with the 13C isotope dependence localization length of single-layer graphene. We noticed that a typical localization length is on the order of ∼1 nm at 70% isotope concentrations. The combined effect of 13C isotope and vacancies shows an abruptly decreasing localization length with increasing defect densities. These results are important to understand the heat conduction as well as nanoscopic vibrational studies such as tip-enhanced Raman spectra in carbon nanotubes where the local phonon energies may be mapped.

Counterintuitive effects of isotopic doping on the phase diagram of H2–HD–D2 molecular alloy

Molecular hydrogen forms the archetypical quantum solid. Its quantum nature is revealed by behavior which is classically impossible and by very strong isotope effects. Isotope effects between [Formula: see text], [Formula: see text], and HD molecules come from mass difference and the different quantum exchange effects: fermionic [Formula: see text] molecules have antisymmetric wavefunctions, while bosonic [Formula: see text] molecules have symmetric wavefunctions, and HD molecules have no exchange symmetry. To investigate how the phase diagram depends on quantum-nuclear effects, we use high-pressure and low-temperature in situ Raman spectroscopy to map out the phase diagrams of [Formula: see text]-HD-[Formula: see text] with various isotope concentrations over a wide pressure-temperature (P-T) range. We find that mixtures of [Formula: see text], HD, and [Formula: see text] behave as an isotopic molecular alloy (ideal solution) and exhibit symmetry-breaking phase transitions between phases I and II and phase III. Surprisingly, all transitions occur at higher pressures for the alloys than either pure [Formula: see text] or [Formula: see text] This runs counter to any quantum effects based on isotope mass but can be explained by quantum trapping of high-kinetic energy states by the exchange interaction.

Atomistic simulations of phonon behaviors in isotopically doped graphene with Sierpinski carpet fractal structure

Two-dimensional (2D) graphene monolayer has been attached importance because of the fantastic physical properties. In this work, we conduct the atomistic simulations to evaluate the phonon behaviors in isotopically doped graphene with Sierpinski Carpet (SC) fractal structure. The thermal conductivities (k) with different fractal numbers are calculated by molecular dynamics simulation. The relationship between the k and the fractal number from 0 to 8 shows a first decreasing and then stable trend. The maximum reduction ratio of the k in SC fractal structures is 52.37%. Afterwards, we utilize the molecular dynamics simulation, phonon wave packet simulation and lattice dynamics simulation to investigate the phonon density of states (PDOS), energy transmission coefficient (ETC), phonon group velocity and participation ratio (PR) in SC fractal structures. In SC fractal structures, the PDOS increases in the low frequency region and the G-band will soften with the enhanced fractal number. We also observe that the isotopic doping atoms can lead to continuous reflected waves in SC fractal structure regions. Moreover, phonon modes in SC fractal structures possess the lower ETCs, phonon group velocities and PRs in comparison with the pristine graphene monolayer. Therefore, we attribute the lower k in SC fractal structures to the stronger phonon-impurity scattering and the increasing localized phonon modes.

Phonon thermal conductivity reduction in silicene nanotubes with isotope substitution.

We used molecular dynamics simulations to study the isotopic doping effects on phonon thermal conductivity in armchair silicene nanotubes (SNTs). The phonon thermal conductivity of armchair SNTs can be effectively tuned with isotope substitution. Randomly and superlattice-structured isotopic doping can significantly reduce thermal conductivity. By analyzing the phonon vibrational spectrum, we reveal the underlying physical insights into the relationship between randomly isotopic doping concentration and thermal conductivity. Given the same doping concentration, the superlattice-structured doping method can reduce thermal conductivity more significantly than the disordered doping. For the isotopic superlattice doping method, the completion between the phonon interfacial scattering and phonon tunneling may cause minimum thermal conductivity at the critical period length. This study provides a possible means to effectively reduce the thermal conductivity of thermoelectric SNTs through isotopic doping engineering.

Tunable thermal transport in a WS2 monolayer with isotopic doping and fractal structure.

An emerging two-dimensional (2D) tungsten disulfide (WS2) has been attracting much attention due to its excellent physical properties. In this work, the thermal transports in isotopically doped WS2 monolayers modified with Rectangle Carpet (RC) and Sierpinski Carpet (SC) structures are investigated systematically using molecular dynamics simulations. The effects of fractal number and temperature on thermal conductivity (k) are evaluated. The SC fractal structures have lower thermal conductivities in comparison with the RC structures for each fractal number. Furthermore, an initial decreasing and then increasing trend of k is observed with increasing fractal number. We can also observe a negative correlation between k and temperature. The phonon dispersion, group velocity, participation ratio, spatial distribution of energy and phonon density of states are evaluated to reveal the thermal transport mechanism in WS2 monolayers. This work provides valuable information on phonon behavior to tune the thermal transport in 2D WS2 monolayers based thermoelectric applications.

Measuring reaction rates at equilibrium with the isotope doping method

Since the time of J. H. van’t Hoff [1], it has been known that chemical equilibrium is dynamic, meaning that at equilibrium, chemical reactions do not cease, but instead the forward and backward reaction rates are equal. The constant concentrations at equilibrium preclude the use of concentrations to measure reaction rates at equilibrium. Therefore, with the exception of a few special cases, no reaction rates at equilibrium have been published in the literature of chemistry, physics, or chemical engineering. Here we report dissolution and precipitation rates at equilibrium for quartz and barite with the isotope-doping method. Experimental data show that dissolution and precipitation rates are equal at equilibrium, indicating the principle of detailed balance (PDB) appear to be applicable at these experimental conditions. The PDB has been a cornerstone for irreversible thermodynamics and chemical kinetics for a long time, and its wide application in geochemistry has mostly been implicit and without experimental testing of its applicability. Nevertheless, many extrapolations based on PDB without experimental validation have far reaching impacts on society’s mega environmental enterprises. The isotope doping method appears to able to test its applicability for a variety of minerals at a wide range of conditions.

Phonon thermal transport in semi-disordered isotopic-superlattice graphene nanoribbons

ABSTRACTThe thermal conductance of isotopic-superlattice graphene nanoribbons randomly mixed with isotope defects is investigated by atomistic Green’s function method. The isotopic-superlattice structure reduces the thermal conductance, which changes non-monotonically as the superlattice period length decreases, and there exists a minimum thermal conductance at the critical period length. About 20% defects just further increase the reduction and shorten the critical period length. As the defect concentration in superlattice increases, the thermal conductance decreases monotonically in the zigzag nanoribbon, but in armchair nanoribbon the thermal conductance first decreases to its minimum value and then increases. The isotopic doping effect is particularly evident for large isotope mass, in armchair nanoribbons and for out-of-plane phonon modes. Isotope defects lead to additional phonon scattering in the superlattice structure, which is explained by analyzing the phonon transmittance.

Supermultiplexed optical imaging and barcoding with engineered polyynes.

Optical multiplexing impacts widely in photonics, life science, biomedicine and engineering. Despite intensive efforts, current technology is limited by a longstanding “multiplexing ceiling” from existing optical materials. Here we engineered a novel class of polyyne-based materials for optical super-multiplexing. 20 distinct Raman frequencies are achieved as “Carbon rainbow” through rational engineering of conjugation length, bond-selective isotope doping and end-capping substitution of polyynes. With further probe functionalization, we demonstrated unprecedented 10-color organelle imaging in single living cell with high specificity, sensitivity, and photo-stability. Moreover, optical data storage and identification are realized by combinatorial barcoding, yielding the largest number of distinct spectral barcodes to date. Therefore, these versatile polyynes hold great promises in live-cell imaging and sorting, high-throughput diagnostics and screening, and information technology.

Effect of isotope doping on phonon thermal conductivity of silicene nanoribbons: A molecular dynamics study**Project supported by the National Natural Science Foundation of China (Grant Nos. 11504418 and 11447033), the Natural Science Fund for Colleges and Universities in Jiangsu Province, China (Grant No. 16KJB460022), and the Fundamental Research Funds for the Central Universities of CUMT, China (Grant No. 2015XKMS075).

Silicene, a silicon analogue of graphene, has attracted increasing research attention in recent years because of its unique electrical and thermal conductivities. In this study, phonon thermal conductivity and its isotopic doping effect in silicene nanoribbons (SNRs) are investigated by using molecular dynamics simulations. The calculated thermal conductivities are approximately 32 W/mK and 35 W/mK for armchair-edged SNRs and zigzag-edged SNRs, respectively, which show anisotropic behaviors. Isotope doping induces mass disorder in the lattice, which results in increased phonon scattering, thus reducing the thermal conductivity. The phonon thermal conductivity of isotopic doped SNR is dependent on the concentration and arrangement pattern of dopants. A maximum reduction of about 15% is obtained at 50% randomly isotopic doping with30Si. In addition, ordered doping (i.e., isotope superlattice) leads to a much larger reduction in thermal conductivity than random doping for the same doping concentration. Particularly, the periodicity of the doping superlattice structure has a significant influence on the thermal conductivity of SNR. Phonon spectrum analysis is also used to qualitatively explain the mechanism of thermal conductivity change induced by isotopic doping. This study highlights the importance of isotopic doping in tuning the thermal properties of silicene, thus guiding defect engineering of the thermal properties of two-dimensional silicon materials.

High thermoelectric and electronic performance in graphene nanoribbons by isotope and vacancy engineering

We study the effects of isotope doping and vacancies on the electronic, phononic and thermoelectric properties of graphene nano-ribbons by means of atomistic simulation based on semi-empirical Tight Binding and Force Constant models combined with the Non-Equilibrium Green’s Function formalism of transport. We demonstrate that from the phononic perspective, the impact of isotope doping on transmission is very dependent on the phonon frequency and particularly strong at high frequency, while vacancies wield their influence on the whole frequency range. From the electronic point of view, if placed at dimer line positions m = 3i + 1, 3i + 2, vacancies induce a strong reduction of the electrical conductance while vacancies at positions m = 3i act as isotopes, i.e. they are transparent to electrons and do not affect the electronic properties. The thermoelectric study is conducted by combining both electronic and phononic properties. It demonstrates the possibility of a remarkable enhancement of figure of merit ZT. In the presence of both types of defects, ZT can be enhanced significantly from 0.26 in pristine ribbons to values larger than 2.5 for ribbons of width M = 5.

Thermal transport across solid-solid interfaces enhanced by pre-interface isotope-phonon scattering

Thermal transport across solid interfaces can play critical roles in the thermal management of electronics. In this letter, we use non-equilibrium molecular dynamics simulations to investigate the isotope effect on the thermal transport across SiC/GaN interfaces. It is found that engineered isotopes (e.g., 10% 15N or 71Ga) in the GaN layer can increase the interfacial thermal conductance compared to the isotopically pure case by as much as 23%. Different isotope doping features, such as the isotope concentration, skin depth of the isotope region, and its distance from the interface, are investigated, and all of them lead to increases in thermal conductance. Studies of spectral temperatures of phonon modes indicate that interfacial thermal transport due to low-frequency phonons (&amp;lt; 20 THz) is enhanced after isotopes are introduced. These results suggest that the enhanced thermal conductance is related to the isotope-phonon scattering, which facilitates the redistribution of phonon energy among different modes to favor a better overall interfacial thermal transport. This work may provide insights into interfacial thermal transport and useful guidance to practical material design.

The water-carbon dioxide miscibility surface to 450 °C and 7 GPa

Fluid-fluid immiscibility in the water-carbon dioxide binary system has been measured to a pressure of 7 GPa and temperature of 450 °C, providing the first such data under conditions overlapping those found in subduction zones and the icy worlds of the outer solar system. Water and carbon dioxide were loaded as inhomogeneous mixtures in externally-heated diamond-anvil cells. Mole fractions were determined by isotopic doping of the initial constituents followed by measurement of the equilibrated isotopic content of the decanted samples. Homogenization and de-homogenization of fluids were observed visually for mole fractions of carbon dioxide from 20 to 90 percent. The recorded pressure-temperature points of homogenization were inverted to give a miscibility surface. The path of the critical curve continues to the highest pressures measured, with no indication of an upper critical endpoint. For mole fractions of carbon dioxide less than 40 percent, the minimum temperature necessary to form a homogeneous solution decreases sharply above a compositionally-dependent pressure. Based on measured intensities of Raman lines, this behavior is linked to a change in speciation as the equilibrium shifts from dissolved carbon dioxide to, presumably, either bicarbonate ion or carbonic acid. Published equations of state give poor representations of the miscibility surface, neither the abrupt behavior due to speciation nor the critical curves being correctly predicted. The experimental constraints provided by the current data should allow useful refinements of equations of state over a wide pressure-temperature-compositional space where only extrapolations and molecular dynamic simulations were previously available. Improved equations of state are a prerequisite for better geochemical models for Earth and extraterrestrial ocean worlds.

Cuprate High Temperature Superconductors and the Vision for Room Temperature Superconductivity

Superconducting transition temperatures of 164 K in cuprate high temperature superconductors (HTS) and recently 200 K in H3S under high pressure encourage us to believe that room temperature superconductivity (RTS) might be possible. In considering paths to RTS, we contrast conventional (BCS) SC, such as probably manifested by H3S, with the unconventional superconductivity (SC) in the cuprate HTS family. Turning to SC models, we show that in the presence of one or more van Hove singularities (vHs) near the Fermi level, SC mediated by classical phonons ([Formula: see text]phonon frequency) can occur. The phonon frequency in the standard [Formula: see text] formula is replaced by an electronic cutoff, enabling a much higher [Formula: see text] independent of phonon frequency. The resulting [Formula: see text] and isotope shift plot versus doping strongly resembles that seen experimentally in HTS. A more detailed theory of HTS, which involves mediation by classical phonons, satisfactorily reproduces the chief anomalous features characteristic of these materials. We propose that, while a path to RTS through an H3S-like scenario via strongly-coupled ultra-high frequency phonons is attractive, features perhaps unavailable at ordinary pressures, a route involving SC mediated by classical phonons which can be low frequency may be found.

Temperature and isotope effects on the thermoelectric properties in SnTe

The temperature and Sn isotope effects on SnTe are investigated using the hybrid scheme that combines ab initio molecular dynamics and lattice dynamics (AIMD + LD). The unstable softening of TA phonon mode at 0 K in cubic phase SnTe disappears and the TO mode is stiffened by considering the temperature effect, in agreement with the inelastic neutron scattering (INS) observations. A linear dependence on isotope mass of phonon frequency, lifetime, and mean free path for isolate phonon mode is observed with the possibility of positive, negative, and nearly zero shifts. The lattice thermal conductivity () shows saturation characteristic as Sn isotope mass increases to 120SnTe, with an increase rate of ∼2.1% from 112SnTe to 124SnTe. Considering the effects of partial isotope doping, we obtain a reduced with respect to the undoped case. The is reduced by ∼14.6% at 600 K when considering the volumetric expansion.

Thermal transport in graphene with defect and doping: Phonon modes analysis

The effects of defect and isotopic doping with different ratios on the thermal conductivity of graphene are investigated by using non-equilibrium molecular dynamics simulations and normal mode analysis method. In contrast to the persisted size dependent thermal conductivity in the pristine graphene, thermal conductivity of defected graphene quickly saturates when the size is greater than 100 nm. Similar to the pristine graphene, we find the thermal conductivity of defected and doped graphene follows ∼T−α behavior, and the power exponent α is sensitive to the defect and doping ratio. The spectral phonon relaxation time and normalized accumulation thermal conductivity with respect to the phonon mean free path (MFP) reveal that the long-MFP phonon modes are strongly suppressed in the defected and doped graphene, resulting in the suppressed size dependence and the weaker temperature dependence of the thermal conductivity compared to the pristine graphene. The phonon modal analysis in our work establishes a deep understanding of the defect and doping effects on the thermal transport in graphene, which would provide effective guidance to the graphene-based phonon engineering applications.