Thermopower and conductance of single-molecule junctions and atomic contacts

Tesis doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física de la Materia Condensada. Fecha de lectura: 11-12-2014Thermoelectric fundamental are problem of dissipation problem materials difficult everyday life. they are light, flexible and potentially needs to be improved. the the most important open problems in nanoscience c optimization nanoscale Single excellent interface at a fundamental level. the electronic conductance is typically the only magnitude measured. recently the transport process has been demonstrated just a few groups The main conductance of single (STM) new STM head specifically designed for these measurements and the development of a no and applied believed to waste heat recovery (e.g. from transportation vehicles) or the heat materials, despite the good performance to process Organic thermoelectric materia introduction of nanostructure ptimization of thermoelectricity [Zhang2014 Single-molecule junctions formed using scanning probe techniques constitute an model system to study the processes occurring at the organic recently, the possibilit goal of this thesis has been to in ambient conditions. An important part of this work is the construction of a novel powerful technique for measuring effects in molecular junctions are point of view be one of the potential solutions for key energy problems like the (e.g. in microelectronics). , (energetically expensive and toxic), heavy and brittle for use in A strategy for enhanc ntroduction Zhang2014]. possibility of measuring the thermopower to give further insight into [Baheti2008 single-molecule junctions using vel view. cheap, nanostructures and multiple interfaces in organic thermoelectric materials In most of the experiments in molecular junctions, y Baheti2008; Widawsky2011 study experimentally the thermo Indeed, organic thermoelec Present day inorganic thermoele performance, are already globally limited, relatively materials are promising alternatives although their present efficiency still enhancing the thermoelectric performance is concerns the understanding [Reddy2007 Widawsky2011; Yee2011 a scanning tunneling m simultaneously of great interest f rganic resent ls lthough [See2010 oncerns Reddy2007] and is currently use Yee2011]. the thermopower Abstract thermoelectric materials since See2010]. Thus, one of at the organic-inorganic Quite thermopower and microscope 5 from thermoelectric and used by er icroscope and conductance of single-molecule junctions, making a complete characterization of the molecular junction possible. This is detailed in chapter 4. In chapter 5, this new technique is used to measure the thermopower of C60 molecules and demonstrate the possibility of engineering the thermopower of a molecular junction by molecular scale manipulation, in particular, the enhancement of thermopower by forming a C60 dimer is shown. The thermoelectric properties of atomic nanocontacts of gold and platinum are explored in chapter 6. As contact size dimensions are reduced, a crossover from bulk to quantum behaviour involving a change of sign of the thermopower takes place. Interestingly, quantum oscillations are observed in gold atomic-size contacts, whereas in platinum they are totally absent. This difference between gold and platinum is traced back to the different electronic structure of these two metals. In chapter 7 the effect of lateral chains on the thermopower of OPE derivatives is examined. The addition of lateral chains is found to increase the thermopower as it brings the Fermi level closer to molecular resonances. An enhancement of thermopower with stretching of the molecule is also observed. Finally, in chapter 8 the use of C60 as a linker in molecular junctions is explored by forming single-molecule junctions of dumbbell molecules, consisting of two fullerenes joined by a conjugated backbone.This work has been supported by the European Union (FP7) through programs ITN “FUNMOLS” Project Number 212942, ELFOS and by the A.G. Leventis Foundatio

Novel nanoscale method for thermal conductivity measurements

As the downscaling of electronic devices pushes dimensions of its components towards the atomic limits, new measurement tools need to be developed to address new challenges. In particular, nanoscale heat transfer is a key mechanism which is known to limit the performance of nanoscale sized transistors in the processor chips and thus invalidate of a major component Moore’s law of the processor speed increase [1]. Measurements of thermal conductivity for simple geometry such as thin films present many difficulties to traditional techniques for layer thicknesses smaller than 100 nm [2]. For example, decoupling the thermal conductivity from the interfacial resistances between the film and the substrate as well as the probe and the film is often difficult. In this report, we develop a novel approach addressing these challenges. We combine a unique cross-sectional tool and a heated probe – scanning thermal microscopy, or SThM, we were able to measure intrinsic thermal conductivity of few tens of um thin layer-on-substrate and to deduce the interfacial thermal resistance. Beam-exit cross-sectional polishing (BEXP) uses Ar-ion beams impinging on a sample at shallow angle (<10 ) [3,4]. The cross-sectioned surface obtained has preferential geometry and sub-nm surface roughness making it easily suitable for studies via standard scanning probe microscopy methods (Fig. 1). Nanothermal microscopy techniques are gaining interest as they resolve thermal properties below the diffraction limit [5,6]. SThM uses the atomic force microscopy principles to raster a thermosensitive probe on a surface. The electrical resistance of the probe is monitored as it scans the sample and by relating this resistance with the temperature, heat transfer properties of the sample can be deduced [7]. To validate our approach, we apply this method on different commonly used materials from semiconductors to insulators such as silicon dioxide, spin-on-glass and spin-onpolymers. The BEXP cross-sectioning process enables the measurements of the SThM response as a function of the layer thickness (Fig. 2). By analysing the SThM signal of the wedge-shaped section of the probed material, we were able to extract the thermal conductivity of the layer itself by combining analytical and finite element modelling of the sample. The thermal conductivity and the interfacial thermal resistance, which is a big unknown for all these materials, can be directly obtained by fitting the measurements of the thermal resistance as a function of the position of the probe, to our model. We confirm capabilities of this new method for standard materials using different modelling approaches. Our results demonstrate its applicability for direct measurements of otherwise hard to obtain quantities for previously unknown materials. The ease of use of our method renders it suitable for a broad range of samples and opens new paths for fundamental and applied research in wide areas from biology to spintronics. Acknowledgements The authors acknowledge the support of project EU FP7-NMP-2013-LARGE-7 QuantiHea

Scanning Thermal Microscopy on 2D Materials at cryogenic temperatures

Thermal transport in Graphene is of great interest due to its high thermal conductivity, for both fundamental research and future applications such as heat dissipation in electronic devices. Although, the thermal conductivity of graphene can reduce depending on the coupling to the substrate [1]. In this work, we report high-resolution imaging of nanoscale thermal transport in single and few layers of Graphene on Silicon Oxide (SiO2) and hexagonal Boron Nitride (hBN), by Scanning Thermal Microscopy (SThM) in high vacuum. SThM is a leading technique for mapping thermal properties with nanoscale resolution [2], consisting of a self-heated probe which acts as a thermosensor during sample scanning. By using doped Si probes and cooling the sample down to 150K,we mapped the thermal resistance of Graphene layers on SiO2 and hBN with sub-10nm resolution. We observed that thermal transport in these layers changes at the elastically deformed areas, which were formed during deposition in the form of bubbles [3]. More specifically, the thermal conductance at the center of the bubbles increases with their surface area. In addition, we study the effect of the sample temperature and the substrate on the thermal conductance of the graphene layers

Characterisation of local thermal properties in nanoscale structures by scanning thermal microscopy

Local characterisation of material thermal properties has become increasingly relevant, but also increasingly challenging, as the size of thermally-active components has been reduced from the micro- to the nano-scale [1] such as in devices based on semiconductor quantum dots and quantum wells, polymer nanocomposites, multilayer coatings, nanoelectronic and optoelectronic devices. In this scenario, thermal management arises as one of the main issues to be treated as the proximity of interfaces and the extremely small volume of heat dissipation strongly modifies thermal transport and imposes a limit on the operation speed and the reliability of the new devices [2]. It therefore becomes critical to fully characterise the local nanoscale heat transport properties of different materials currently used in various industrial applications such as semiconductors, insulators, polymers etc, operating under different conditions and with varying doping levels [3]. Specifically, silicon is of interest due to its ubiquity in most sensors, electronic components or photovoltaic cells. In the present study, we compare doped and intrinsic semiconductor to polymeric sample that have been characterised both topographically and thermally by means of scanning thermal microscopy (SThM). Thermal characterisation of the samples was performed with a modified AFM system (NT-MDT Solver) in ambient conditions using a commercial probe with Pd microfabricated resistive heater and custom electronics allowing the measurement of local heat transport between the apex of the probe and the sample [4]. We demonstrate this approach on the set of the reference materials samples of sufficiently large size to be independently measured using standard thermal conductivity methods [5]. In order to improve the quality of the SThM measurements, sample temperature was stabilised via a combination of a Peltier heater mounted underneath the sample and thermistors monitoring the temperature of the sample in a closed loop setup, with the temperatures of the probe base and surrounding air continuously monitored. The setup allowed us to simultaneously acquire topographical and thermal measurements in the contact mode. During the measurements, approach-retraction curves (as shown in Figure 1), were taken at 16 different points of the sample’s surface. The SThM electronics produced a voltage output (“thermal signal”) due to the change of the probe resistance proportional to the change in the probe temperature. Probe response is best represented as where is the thermal signal of the probe when it is not in contact with the sample, and is thermal signal when it establishes contact with the surface. This ratio is shown to be directly related to the thermal conductivity of the samples [4]. Our results for the 4 different materials – intrinsic, p++ and n++ doped Si, as well as the polymer are shown in Fig.2. In the measurement conditions of ambient pressure and temperature, single crystalline Si [100] is showing the highest value of the thermal conductivity, with the doped Si species showing lower thermal conductivity with smaller values DV/V, due to phonon-electron scattering that are dominating on the nanoscale [6]. Our measurements show that the SThM can reliably discriminate between group IV semiconductors presenting different doping concentrations based on the thermal conductivity, with a lateral resolution of about 20-50 nm. Further steps will focus on obtaining quantitative data from the DV/V measurements, using for this purpose, specially prepared reference samples of controlled geometry that can be characterised independently via large scale techniques such as flash thermoreflectance [5]

Charge-state dependent vibrational relaxation in a single-molecule junction

The interplay between nuclear and electronic degrees of freedom strongly influences molecular charge transport. Herein, we report on transport through a porphyrin dimer molecule, weakly coupled to graphene electrodes, that displays sequential tunneling within the Coulomb-blockade regime. The sequential transport is initiated by current-induced phonon absorption and proceeds by rapid sequential transport via a non-equilibrium vibrational distribution. We demonstrate this is possible only when the vibrational dissipation is slow relative to sequential tunneling rates, and obtain a lower bound for the vibrational relaxation time of 8 ns, a value that is dependent on the molecular charge state.Comment: 8 pages, 7 figure

Structural Characterisation of ALD coated Porous Si via Beam-Exit Cross-Sectional Polishing

Porous silicon (PS) samples with aspect ratios approaching 1:600 were conformally coated with Al O using atomic layer deposition (ALD). Beam-exit cross-sectional polishing (BEXP) was used to create shallow-angled cross-sections of the ALD coated samples, facilitating the study of the internal structure of the PS via scanning probe microscopy (SPM). ALD coating was found to be conformal along much of the pore height, although pores were observed to become blocked closer to the sample surface. Conformal coatings of materials have played an important role in the development and production of a wide range of devices including insulators, conductors, diffusion barriers and adhesive layers. The application of conformal coatings in high aspect ratio (HAR) structures, such as super-capacitors, transistor channels, memory applications, catalytic membranes, biomedicine and gas sensors show great potential due to the extensive surface area compared with 2D structures [1-3]. Porous silicon (PS) is a promising candidate for a number of applications as it relatively easy to prepare and offers large surface area. Unfortunately, the consistent coating of HAR structures presents difficulties in maintaining coverage and conformality of functional layers. ALD allows the deposition of a wide range of conducting and isolating materials with conformality and layer thickness control. The ALD coating of HAR structures requires optimisation and characterisation of coated structures can be of great benefit to the process. Scanning electron microscopy (SEM) observation of layers can be a straightforward way of checking coatings for some structures, but can encounter difficulties when imaging thin layers or material combinations that provide low contrast. Alternative techniques exist, such as exploiting the resistance of Al O against SF /O plasma in deep reactive ion etching (DRIE) of silicon, can involve etching away the porous frame of a HAR sample [4], allowing the subsequently revealed ALD layers to be studied. This work presents the results of an alternative method of studying ALD coated PS. The BEXP technique allows the internal structure of a sample to be investigated using SPM techniques by producing a shallow-angled cross-section through the sample, usually 5 - 12° [5]. Crucially, this particular arrangement means that the area of interest is exposed to the Ar –ion beam-exit, rather than the beam-entry as in standard Ar-ion milling, producing cross-sections with sub-nm roughness and extremely low amounts of damage, ideal for SPM analysis

Nanoscale Thermal Transport in 2D Nanostructures from Cryogenic to Room Temperature

Nanoscale scanning thermal microscopy (SThM) transport measurements from cryogenic to room temperature on 2D structures with sub 30 nm resolution are reported. This novel cryogenic operation of SThM, extending the temperature range of the sample down to 150 K, yields a clear insight into the nanothermal properties of the 2D nanostructures and supports the model of ballistic transport contribution at the edge of the detached areas of exfoliated graphene which leads to a size-dependent thermal resistance of the detached material. The thermal resistance of graphene on SiO2 is increased by one order of magnitude by the addition of a top layer of MoS2, over the temperature range of 150–300 K, providing pathways for increasing the efficiency of thermoelectric applications using van der Waals (vdW) materials. Density functional theory calculations demonstrate that this increase originates from the phonon transport filtering in the weak vdW coupling between the layers and the vibrational mismatch between MoS2 and graphene layers

Efficient heating of single-molecule junctions for thermoelectric studies at cryogenic temperatures

The energy dependent thermoelectric response of a single molecule contains valuable information about its transmission function and its excited states. However, measuring it requires devices that can efficiently heat up one side of the molecule while being able to tune its electrochemical potential over a wide energy range. Furthermore, to increase junction stability, devices need to operate at cryogenic temperatures. In this work, we report on a device architecture to study the thermoelectric properties and the conductance of single molecules simultaneously over a wide energy range. We employ a sample heater in direct contact with the metallic electrodes contacting the single molecule which allows us to apply temperature biases up to ΔT = 60 K with minimal heating of the molecular junction. This makes these devices compatible with base temperatures Tbath < 2 K and enables studies in the linear (Δ T ≪ T molecule) and nonlinear (Δ T ≫ T molecule) thermoelectric transport regimes

Nanoscale mapping of thermal and mechanical properties of bare and metal-covered self-assembled block copolymer thin films

We report on the structural, mechanical and thermal analysis of 40 nm thick polystyrene-block-poly (ethylene oxide) (PS-b-PEO) block copolymer (BCP) films coated with evaporated chromium layers of different thicknesses (1, 2 and 5 nm). Solvent annealing processes allow the structural control of the BCP films morphology by re-arranging the position of the PEO cylinders parallel to the substrate plane. High-vacuum scanning thermal microscopy and ultrasonic force microscopy measurements performed in ambient pressure revealed that coated ultrathin metal layers strongly influence the heat dissipation in the BCP films and the local surface stiffness of the individual BCP domains, respectively. The measured tip-sample effective thermal resistance decreases from 6.1×107 to 2.5×107 KW-1 with increasing Cr film thickness. In addition, scanning probe microscopy measurements allow the thermal and mechanical mapping of the two segregated polymer domains (PEO-PS) of sub-50 nm characteristic sizes, with sub-10 nm thermal spatial resolution. The results revealed the effect of the surface morphology of the BCP and the incorporation of the metal film on the nanoscale thermal properties and volume self-assembly on the mechanical properties. The findings from this study provide insight in the formation of high aspect ratio BCP-metal structures with the more established applications in lithography. In addition, knowledge on the thermal and mechanical properties at the nanoscale is required in emergent applications, where BCPs, or polymers in general, are part of the structure or device. The performance of such devices is commonly related to the requirement of increased heat dissipation while maintaining mechanical flexibility

Invited talk - Nanoscale thermal transport and unconventional thermoelectric phenomena in 2D materials

With 2D materials such as graphene (GR) and hexagonal boron nitride possessing highest known thermal conductivities, one-atom thick nature of these materials makes thermal transport in them drastically dependent on the local environment. Moreover, the equally extraordinary electronic properties of GR such as relativistic carrier dynamics combined with GR highly anisotropic thermal conductance may point to unusual thermoelectric properties. In order to study thermal and thermoelectric phenomena in these nanoscale materials, we applied scanning thermal microscopy (SThM) that uses a sharp tip in contact with the probed surface that can create a controlled local sample temperature rise in the few nm acros spot, while measuring the resulting sample temperature and a heat flow. We used high vacuum environment that eliminates spurious heat dissipation channels to boost accuracy and sensitivity and to allow cryogenic measurements. We show that the thermal resistance of GR on SiO2 is increased by one order of magnitude by the addition of a top layer of MoS2, over the temperature range 150- 300 K with DFT calculations attributing this increase to the phonon transport filtering in the weak vdW coupling and vibrational mismatch between dissimilar 2D materials. By measuring the heat generated in the nanoscale constrictions in monolayer GR devices, we have discovered unconventional thermoelectric Peltier effect due to geometrical shape of 2D material and not requiring a junction of dissimilar materials, with phenomenon confirmed by measuring the Seebeck thermovoltage map due to local heating by the SThM tip. The novel nonlinear thermoelectric phenomena due to “electron wind”, and effects of GR doping and layer number are also reported