Isotope Doping Research Articles

Some Aspects of Thermal Transport across the Interface between Graphene and Epoxy in Nanocomposites.

Owing to the superior thermal properties of graphene, graphene-reinforced polymer nanocomposites hold great potential as the thermal interface materials (TIMs) dissipating heat for electronic packages. However, this application is greatly hindered by the high thermal resistance at the interface between graphene and polymer. In this paper, some important aspects of the improvement of the thermal transport across the interface between graphene and epoxy in graphene-epoxy nanocomposites, including the effectiveness of covalent and noncovalent functionalization, isotope doping, and acetylenic linkage in graphene are systematically investigated using molecular dynamics (MD) simulations. The simulation results show that the covalent and noncovalent functionalization techniques could considerably reduce the graphene-epoxy interfacial thermal resistance in the nanocomposites. Among different covalent functional groups, butyl is more effective than carboxyl and hydroxyl in reducing the interfacial thermal resistance. Different noncovalent functional molecules, including 1-pyrenebutyl, 1-pyrenebutyric acid, and 1-pyrenebutylamine, yield a similar amount of reductions. Moreover, it is found that the graphene-epoxy interfacial thermal resistance is insensitive to the carbon isotope doping in graphene, while it can be reduced moderately by replacing the sp(2) bonds in graphene with acetylenic linkages.

Measuring silicate mineral dissolution rates using Si isotope doping

New experimental data and quantitative models show that the 29Si doping experimental technique (Gruber, Zhu, and others, 2013, GCA) is robust for measuring silicate mineral dissolution rates even while a Si-containing secondary phase is precipitating. In this study, batch experiments of albite dissolution were conducted under ambient temperature and pH3–7.5, some seeded with kaolinite. Initial solutions of various Si concentrations were doped with 29Si, resulting in a Si isotopic composition highly anomalous to natural Si isotope compositions. The isotopic contrast and precision of isotope fraction analysis to ±0.0005 to ±0.001 allow detection of the dissolution of a minuscule amount of albite in aqueous solutions. Experimental data and quantitative modeling show Si isotope fractionation during albite dissolution ranged from 30εsol-ab −2.870 to 0.804‰, significant for Si biogeochemical cycling, but resulting in only <±0.04% errors in rate determination. The simultaneous precipitation of secondary phases consumed silica, causing slight changes of Si isotope ratios, but the isotopic fractionation due to secondary phase precipitation is negligible for determining albite dissolution rates. Combination of Si isotopes and Si concentrations, precisely measured with the Si isotope dilution method, allowed determination of secondary phase precipitation rates simultaneously. This means that we can now measure rates at circumneutral pH and near equilibrium conditions, even when secondary precipitates are forming. However, while the isotope doping method has greatly improved the precision and sensitivity of rate measurements, the accuracy of rate measurements is still subject to the vagaries of sample preparation and other unknown effects as shown our data near pH5.5. When the solution is very close to equilibrium, the backward reaction becomes important and interpretation of the isotope data would be complicated or impossible.

Molecular dynamics simulations of thermal transport in isotopically modulated semiconductor nanostructures

AbstractIn this paper, we investigate the effect of isotopic modulation on the thermal conductivity of semiconductor nanostructures. The isotope doping is of particular interest for the application of semiconductors as thermoelectric materials as it leaves the electronic properties practically unaffected while the phononic transport is retarded. This approach could increase the figure of merit of thermoelectric generators by decreasing the thermal conductivity of semiconductors. We use non‐equilibrium molecular dynamics simulations to examine thermal transport in isotopically engineered semiconductors. The temperature profiles along the sample region deduced from the simulations allow the extraction of thermal conductivities. The reliability of the MD‐predicted thermal conductivities is studied by analyzing the influence of the input parameters on the results. The first set of samples are isotopically modified silicon samples. The influence of temperature, isotopic composition, and ordering of isotopic defects on the thermal conductivity of silicon is studied. The second material system under investigation is silicon germanium alloys. The influence of isotopic modulation on the thermal conductivity of Si–Ge alloys is examined for varying chemical composition. The thermal conductivities predicted by MD are compared to results derived from the solution of the Boltzmann transport equation in the relaxation time approach.

A stable isotope doping method to test the range of applicability of detailed balance

A stable isotope doping method to test the range of applicability of detailed balance

Phonon coherence in isotopic silicon superlattices

Recent experimental and theoretical investigations have confirmed that a reduction in thermal conductivity of silicon is achieved by isotopic silicon superlattices. In the present study, non-equilibrium molecular dynamics simulations are performed to identify the isotope doping and isotope layer ordering with minimum thermal conductivity. Furthermore, the impact of isotopic intermixing at the superlattice interfaces on phonon transport is investigated. Our results reveal that the coherence of phonons in isotopic Si superlattices is prevented if interfacial mixing of isotopes is considered.

Low thermal conductivity in ultrathin carbon nanotube (2, 1).

Molecular dynamic simulations reveal that the ultrathin carbon nanotube (CNT) (2, 1) with a reconstructed structure exhibits a surprisingly low thermal conductivity, which is only ~16–30% of those in regular CNTs, e.g. CNT (2, 2) and (5, 5). Detailed lattice dynamic calculations suggest that the acoustic phonon modes greatly soften in CNT (2, 1) as compared to regular CNTs. Moreover, both phonon group velocities and phonon lifetimes strikingly decrease in CNT (2, 1), which result in the remarkable reduction of thermal conductivity. Besides, isotope doping and chemical functionalization enable the further reduction of thermal conductivity in CNT (2, 1).

Thermal conductivity of silicene nanosheets and the effect of isotopic doping

This paper investigates the thermal conductivity of silicene nanosheets (SiNSs) via molecular dynamic simulation. It shows that the thermal conductivity of pristine SiNSs is about 25–30 W mK−1 and exhibits anisotropic behaviour. Moreover, it is found that isotopic doping is efficient in reducing the thermal conductivity of SiNSs. When SiNSs are randomly doped with 30Si at the doping percentage of 50%, a maximum reduction of about 20% is obtained. This reduction can be increased when the dopants are arranged into a superlattice pattern. The thermal conductivity of these superlattice-structured SiNSs changes non-monotonically as the thickness of their lattice layers decreases. Detailed analysis of the phonon spectra demonstrates that the competing mechanism between the interface scattering and confinement effects of the phonon modes is responsible for this non-monotonical behaviour.

Silicon Isotopes as a New Method of Measuring Silicate Mineral Reaction Rates at Ambient Temperature

Experiments of albite dissolution under ambient temperature at pH 3 and 5 demonstrated the feasibility of using 29Si doping method to precisely measure silicate mineral dissolution rates. Initial solutions were doped with 29Si to result in an isotopic composition of 0.04% 28Si, 99.9% 29Si, and 0.06% 30Si, which compares with the Amelia and Evja albite with Si isotopic composition of 92.23% 28Si, 4.67% 29Si, and 3.10% 30Si. The isotopic contrast allows the detection of dissolution of a tiny amount of albite into the aqueous solution. Experimental results show rapid increase of 28Si abundance at the initial stage of fresh albite dissolution, reflecting reaction with high energy sites on the surface. Steady state dissolution rates were reached after 20-30 days at 22 oC. The evolution of 28Si and 29Si abundances (%) with time tightly constrains the albite dissolution rates, which are consistent with literature data. In experiments seeded with kaolinite, both albite dissolution rates and kaolinite precipitation rates were determined with a combination of Si isotopes and isotope dilution method of determining Si concentrations.The new experiments have also verified the assumptions that are necessary for the isotope doping method to be applicable. Experimental data showed no Si isotope fractionation during albite dissolution. The ion-exchange method in sample pre-treatments does not alter Si isotopic composition or silica concentrations. The errors from neglecting isotope fractionation during secondary mineral precipitation are small on the determined albite dissolution rates when the isotope doping is overwhelming. In conclusion, we now have substantial experimental data show that the 29Si doping is a new method applicable to a wide range of silicate mineral dissolution rate determinations. Particularly, our albite experimental results show promise to use isotope doping method to measure silicate dissolution rates at various saturation states and determine the rate – ΔGr relationships.

Thermal conductivity of isotopically controlled silicon nanostructures

Nanostructured semiconductors open the opportunity to independently tailor electric and thermal conductivity by manipulation of the phonon transport. Nanostructuring of materials is a highly promising strategy for engineering thermoelectric devices with improved efficiency. The concept of reducing the thermal conductivity without degrading the electrical conductivity is most ideally realized by controlled isotope doping. This work reports on experimental and theoretical investigations on the thermal conductivity of isotopically modulated silicon nanostructures. State-of-the-art pump-and-probe experiments are conducted to determine the thermal conductivity of the different nanostructures of isotopically enriched silicon layers epitaxially grown on natural silicon substrates. Concomitant molecular dynamics calculations are performed to study the impact of the silicon isotope mass, isotope interfaces, and of the isotope layer ordering and thickness on the thermal conductivity. Engineering the isotope distribution is a striking concept to reduce the thermal conductivity of silicon without affecting its electronic properties. This approach, using isotopically engineered silicon, might pave the way for future commercial thermoelectric devices.

Tuning the thermal conductivity of silicene with tensile strain and isotopic doping: A molecular dynamics study

Silicene is a monolayer of silicon atoms arranged in honeycomb lattice similar to graphene. We study the thermal transport in silicene by using non-equilibrium molecular dynamics simulations. We focus on the effects of tensile strain and isotopic doping on the thermal conductivity, in order to tune the thermal conductivity of silicene. We find that the thermal conductivity of silicene, which is shown to be only about 20% of that of bulk silicon, increases at small tensile strains but decreases at large strains. We also find that isotopic doping of silicene results in a U-shaped change of the thermal conductivity for the isotope concentration varying from 0% to 100%. We further show that ordered doping (isotope superlattice) leads to a much larger reduction in thermal conductivity than random doping. Our findings are important for the thermal management in silicene-based electronic devices and for thermoelectric applications of silicene.

Thermoelectric Performance of a Single-Layer Graphene Sheet for Energy Harvesting

We investigate the thermoelectric (TE) figure-of-merit of a single-layer graphene (SLG) sheet by a physics-based analytical technique. We first develop analytical models of electrical and thermal resistances and the Seebeck coefficient of SLG by considering electron interactions with the in-plane and flexural phonons. Using those models, we show that both the figure-of-merit and the TE efficiency can be substantially increased with the addition of isotope doping as it significantly reduces the phonon-dominated thermal conductivity. In addition, we report that the TE open circuit output voltage and output power depends weakly on the SLG sheet dimensions and sheet concentration in the strongly diffusive regime. Proposed models agree well with the available experimental data and demonstrate the immense potential of graphene for waste-heat recovery application.

Impact of Isotope Doping on Phonon Thermal Transport in Silicon Nanowires

Phonon transport in silicon nanowires (Si NWs) doped with isotopes is investigated theoretically. The ballistic thermal conductance and diffusive thermal conductivity are calculated at room temperature using the phonon dispersion relations derived through a semiempirical atomistic approach. The thermal conductance and conductivity in 28Si NWs randomly doped with 29Si are smaller than those in the corresponding pure 28Si NW, which can be fully explained by the effect of isotope impurities on the dispersion relations. In [001]-oriented 28Si NWs having a square cross section with a side length of 1.086 nm and randomly doped with 29Si, the maximum reduction in thermal conductivity reaches more than 20%. This reduction leads directly to an improvement in the thermoelectric figure of merit by more than 25%. It is also found that the impact of isotope impurities on phonon transport becomes large with increasing mass difference between the constituent and impurity isotopes or with increasing wire cross-sectional area. Phonon transport in isotopic core–shell Si NWs is also investigated. Some of these Si NWs show increased thermal conductance and conductivity although the increase is very small.

Carbon isotope doping induced interfacial thermal resistance and thermal rectification in graphene

We investigate the thermal transport properties of carbon isotope doped graphene using nonequilibrium molecular dynamics simulations. We find that the interfacial thermal resistance between graphene and the isotope atoms causes severe reduction in thermal conductivity of the doped graphene. Furthermore, we find that thermal rectification occurs in the interface. Tensile strain leads to an increase in the interfacial thermal resistance and thermal rectification, while increasing temperature decreases these parameters. We calculate the phonon spectra and find that the thermal rectification is associated with the overlap areas in the phonon spectra.

A nonequilibrium Green’s function study of thermoelectric properties in single-walled carbon nanotubes

The phonon and electron transport in single-walled carbon nanotubes (SWCNT) are investigated using the nonequilibrium Green’s function approach. In zigzag SWCNT (n,0) with mod(n,3)≠0, the thermal conductance is mainly attributed to the phonon transport, while the electron only has few percentage contribution. The maximum value of the figure of merit (ZT) is about 0.2 in this type of SWCNT. The ZT is considerably larger in narrower SWCNT because of enhanced Seebeck coefficient. ZT is smaller in the armchair SWCNT, where Seebeck coefficient is small due to zero band gap. It is found that the cluster isotopic doping can reduce the phonon thermal conductance obviously and enhance the value of ZT. The uniaxial elongation and compress strain depresses phonons in whole frequency region, leading to the reduction in the phonon thermal conductance in whole temperature range. Interestingly, the elongation strain can affect the phonon transport more seriously than the compress strain, because the high frequency G mode is completely filtered out under elongation strain ϵ&amp;gt;0.05. The strain also has important effect on the subband edges of the electron band structure by smoothing the steps in the electron transmission function. The ZT is decreased by strain as the reduction in the electronic conductance overcomes the reduction in the thermal conductance.

Simulation of phonon transmission through graphene and graphene nanoribbons with a Green’s function method

In this paper, phonon transmission through a graphene sheet and graphene nanoribbons is investigated using an atomistic Green’s function method. Best-fit results from first-principles calculations using a fourth nearest neighbor force-constant model are used to establish the matrices that describe interactions among carbon atoms. The effect of carbon isotopes on thermal conductance is investigated, and the results reveal that isotopic doping moderately reduces both phonon transmission function and thermal conductance. The phonon transmission function of each vibrational branch in the heterogeneous interface is also calculated, and comparisons indicate the major and minor channels of phonon transport through graphene. Further, phonon wave effects in zigzag and armchair edge ribbons are investigated. Phonon transmission functions and thermal conductances are found to be sensitive to the edge shape of structures. The phonon transmission functions of nanoribbons with defects are evaluated by artificially creating mismatches at interfaces. By comparing the transmission function of different defect patterns and the corresponding thermal conductances, the reduction in phonon transport is quantified. The length of defects is found to be important to phonon transport. The results herein offer a useful reference and suggest directions for future research on thermal applications of this material.

Isotopic effects on the thermal conductivity of graphene nanoribbons: Localization mechanism

Thermal conductivity of graphene nanoribbons (GNR) with length 106 Å and width 4.92 Å after isotopic doping is investigated by molecular dynamics with quantum correction. Two interesting phenomena are found, (1) isotopic doping reduces thermal conductivity effectively in low doping region, and the reduction slows down in high doping region, (2) thermal conductivity increases with increasing temperature in both pure and doped GNR, but the increasing behavior is much more slow in the doped GNR than that in pure ones. Further studies reveal that the physics of these two phenomena is related to the localized phonon modes, whose number increases quickly (slowly) with increasing isotopic doping in low (high) isotopic doping region.

Size dependent thermoelectric properties of silicon nanowires

By using first-principles tight-binding electronic structure calculation and Boltzmann transport equation, we investigate the size dependence of thermoelectric properties of silicon nanowires (SiNWs). With cross section area increasing, the electrical conductivity increases slowly, while the Seebeck coefficient reduces remarkably. This leads to a quick reduction of cooling power factor with diameter. Moreover, the figure of merit also decreases with transverse size. Our results demonstrate that in thermoelectric application, NW with small diameter is preferred. We also predict that isotopic doping can increase the value of ZT significantly. With 50% S29i doping (S28i0.5 S29i0.5 NW), the ZT can be increased by 31%.

Determination of Global Protein Turnover in Stressed Mycobacterium Cells Using Hybrid-Linear Ion Trap-Fourier Transform Mass Spectrometry

We determined the global protein turnover profiles for Mycobacterium smegmatis under acid shock and iron starvation conditions using a simple (15)N isotope doping technique and a complete medium replacement method for chasing. We used a high-resolution hybrid-linear ion trap-Fourier transform mass spectrometer coupled with nanoliquid chromatography separation to measure protein turnover values for 151 proteins over a dynamic range of 3 orders of magnitude ranging from about 0.2 to 500. Of these 151 proteins, 31 had significant protein turnover changes (p <0.05) at both stress conditions and had protein turnover values increased or decreased by more than 2-fold under at least one stress condition. Protein turnover increased under acid shock for 28 of the 31 proteins but decreased under iron starvation for all the 31 proteins. Only two proteins had protein turnover lowered by more than 2-fold (p <0.05) under both stress conditions, including an ATP synthase F1 beta subunit (MSMEG4921; AtpD) and a catalase/peroxidase (MSMEG6346; KatG). KatG is required for in vivo activation of isoniazid to be bacterialcidal. Decrease of KatG protein turnover under both stress conditions supports the view that isoniazid may induce a dormancy program in mycobacteria, which in turn limits the efficacy of this drug against dormant subpopulation of mycobacteria. Thus, measuring protein turnover in stressed Mycobacterium cells has implications in understanding drug action and resistance mechanisms.

Effect of electron–phonon interaction on the single-particle spectral properties of the Hubbard model

The effect of electron–phonon interaction on quasiparticle dispersion and spectral density is considered within second order adiabatic approximation of the two-dimensional Holstein–Hubbard model. The electron–phonon interaction gives rise to a kink in the dispersion along the nodal direction of the Brillouin zone whose dependence on isotope substitution and doping is discussed in relation to recent photoemission experiments on the nodal quasiparticles in high-temperature superconductors. It is shown that various experimental features can be qualitatively explained in a framework that considers both strong correlations effect and the usual electron–phonon interaction.

Isotope effect on phonon spectra in single-walled carbon nanotubes

Carbon nanotubes are usually made up of $^{12}\mathrm{C}$ atoms. By adopting the force constant model, the effects of isotope doping with $^{13}\mathrm{C}$ on the vibrational, optical, and thermal properties of single-walled carbon nanotubes are theoretically investigated, including the phonon eigenmodes and density of states, the Raman scattering spectra, and specific heat. Three types of doping patterns (axial stripes, circumferential bands of varying width, and random) are studied, in which an emphasis is put on the isotope-nanotube-junctions. Several new effects of the isotope doping in nanotubes have been found, including the confined optical phonon modes due to the spatial separation of isotopes, the red-shift of Raman peaks in general and blue-shift anomaly of the confined LO peaks in specific tube chirality and doping configuration, and the enhancement of the specific heat due to the $^{13}\mathrm{C}$-doping. This work provides with a theoretical guide for isotope labeling, which is important for understanding the growth mechanism of the carbon nanotubes.