Heiner Linke

Reversible Thermoelectric Nanomaterials

Irreversible effects in thermoelectric materials limit their efficiency and economy for applications in power generation and refrigeration. While electron transport is unavoidably irreversible in bulk materials, here we derive conditions under which reversible diffusive electron transport can be achieved in nanostructured thermoelectric materials. We provide a fundamental thermodynamic explanation for why the optimum density of states in a thermoelectric material is a delta function and for why inhomogeneous doping and segmentation improve the thermoelectric figure of merit.

Reversible quantum brownian heat engines for electrons.

Brownian heat engines use local temperature gradients in asymmetric potentials to move particles against an external force. The energy efficiency of such machines is generally limited by irreversible heat flow carried by particles that make contact with different heat baths. Here we show that, by using a suitably chosen energy filter, electrons can be transferred reversibly between reservoirs that have different temperatures and electrochemical potentials. We apply this result to propose heat engines based on mesoscopic semiconductor ratchets, which can quasistatically operate arbitrarily close to Carnot efficiency.

Thermoelectric efficiency at maximum power in low-dimensional systems

Low-dimensional electronic systems in thermoelectrics have the potential to achieve high thermal-to-electric energy conversion efficiency. A key measure of performance is the efficiency when the device is operated under maximum power conditions. Here we study the efficiency at maximum power, in the absence of phonon-mediated heat flow, of three low-dimensional, thermoelectric systems: a zero-dimensional quantum dot with a Lorentzian transmission resonance of finite width, a one-dimensional (1D) ballistic conductor, and a thermionic (TI) power generator formed by a two-dimensional energy barrier. In all three systems, the efficiency at maximum power is independent of temperature, and in each case a careful tuning of relevant energies is required to achieve maximal performance. We find that quantum dots perform relatively poorly under maximum power conditions, with relatively low efficiency and small power throughput. Ideal one-dimensional conductors offer the highest efficiency at maximum power (36% of the Carnot efficiency). Whether 1D or TI systems achieve the larger maximum power output depends on temperature and area filling factor. These results are also discussed in the context of the traditional figure of merit ZT. (Less)

Power optimization in thermionic devices

Conventional thermionic power generators and refrigerators utilize a barrier in the direction of transport to selectively transmit high-energy electrons. Here we show that the energy spectrum of electrons transmitted in this way is not optimal, and we derive the ideal energy spectrum for operation in the maximum power regime. By using suitable energy filters, such as resonances in quantum dots, the power of thermionic devices can, in principle, be improved by an order of magnitude.Conventional thermionic power generators and refrigerators utilize a barrier in the direction of transport to selectively transmit high-energy electrons, resulting in an energy spectrum of electrons that is not optimal for high efficiency or high power. Here, we derive the ideal energy spectrum for achieving maximum power in thermionic refrigerators and power generators. By using energy barriers that block or transmit electrons according to their total momentum rather than their momentum in the direction of transport, the power of thermionic devices can, in principle, be doubled and the electronic efficiency improved by 25%.

Large Thermoelectric Power Factor Enhancement Observed in InAs Nanowires.

We report the observation of a thermoelectric power factor in InAs nanowires that exceeds that predicted by a single-band bulk model by up to an order of magnitude at temperatures below about 20 K. We attribute this enhancement effect not to the long-predicted 1D subband effects but to quantum-dot-like states that form in electrostatically nonuniform nanowires as a result of interference between propagating states and 0D resonances.

Increasing thermoelectric performance using coherent transport

We show that coherent electron transport through zero-dimensional systems can be used to tailor the shape of the systems transmission function. This quantum-engineering approach can be used to enhance the performance of quantum dots or molecules in thermal-to-electric power conversion. Specifically, we show that electron interference in a two-level system can substantially improve the maximum thermoelectric power and the efficiency at maximum power by suppressing parasitic charge flow near the Fermi energy and by reducing electronic heat conduction. We discuss possible realizations of this approach in molecular junctions or quantum dots.

Thermal Conductance of InAs Nanowire Composites

The ability to measure and understand heat flow in nanowire composites is crucial for applications ranging from high-speed electronics to thermoelectrics. Here we demonstrate the measurement of the thermal conductance of nanowire composites consisting of regular arrays of InAs nanowires embedded in PMMA using time-domain thermoreflectance (TDTR). On the basis of a proposed model for heat flow in the composite, we can, as a consistency check, extract the thermal conductivity Lambda of the InAs nanowires and find Lambda(NW) = 5.3 +/- 1.5 W m(-1) K(-1), in good agreement with theory and previous measurements of individual nanowires.

Nonlinear thermovoltage and thermocurrent in quantum dots

Quantum dots are model systems for quantum thermoelectric behavior because of their ability to control and measure the effects of electron-energy filtering and quantum confinement on thermoelectric properties. Interestingly, nonlinear thermoelectric properties of such small systems can modify the efficiency of thermoelectric power conversion. Using quantum dots embedded in semiconductor nanowires, we measure thermovoltage and thermocurrent that are strongly nonlinear in the applied thermal bias. We show that most of the observed nonlinear effects can be understood in terms of a renormalization of the quantum-dot energy levels as a function of applied thermal bias and provide a theoretical model of the nonlinear thermovoltage taking renormalization into account. Furthermore, we propose a theory that explains a possible source of the observed, pronounced renormalization effect by the melting of Kondo correlations in the mixed-valence regime. The ability to control nonlinear thermoelectric behavior expands the range in which quantum thermoelectric effects may be used for efficient energy conversion.

Concept study for a high-efficiency nanowire based thermoelectric

Materials capable of highly efficient, direct thermal-to-electric energy conversion would have substantial economic potential. Theory predicts that thermoelectric efficiencies approaching the Carnot limit can be achieved at low temperatures in one-dimensional conductors that contain an energy filter such as a double-barrier resonant tunnelling structure. The recent advances in growth techniques suggest that such devices can now be realized in heterostructured, semiconductor nanowires. Here we propose specific structural parameters for InAs/InP nanowires that may allow the experimental observation of near-Carnot efficient thermoelectric energy conversion in a single nanowire at low temperature.

Electron spin resonance investigations of oxidized porous silicon

The defect properties of rapidly thermally oxidized porous silicon are studied by electron paramagnetic resonance. Two different types of defects can be distinguished. One is very similar to the defects observed in damaged crystalline or amorphous Si, whereas the second one is closely related to the Pb center. A maximum defect density of 8×1018 cm−3 is observed for samples annealed at about 600 °C. The intensity of the photoluminescence band at 1.7 eV anticorrelates with the density of the defects.