Quantitative mapping of nanothermal transport via Scanning Thermal Microscopy

By expanding the limit of miniaturisation to achieve superior materials and devices properties, nanoscience and nanotechnology create new challenges to understand materials behaviour at the nanometre length scales. New phenomena require matching tools that in turn open new investigation perspectives, themselves launching platforms for new discoveries. Amongst the vast realm of nanoresearch, thermal properties have gained interest in the past decades due their crucial importance in technological developments, ranging from mainstream semiconductor microelectronics to cutting edge quantum technologies. Whereas major standard techniques to measure thermal properties are not efficient for studying nanosstructured materials, Scanning Thermal Microscopy (SThM) offers both sensitivity to nanoscale thermal transport and a spatial resolution down to a few nanometres. This thesis develops fundamental aspects of the SThM technique by increasing its reproducibility and developing an experimental and analytical framework to analyse experimental data. These developments are then applied to produce quantitative measurements of a wide range of materials from vertically aligned carbon nanotubes to metal covered block copolymers. In brief, we probed the 3D thermal properties distribution of isotropic and anisotropic materials, such as optoelectronic thin film. We also measured low dimensional systems of 2D materials heterostructures from franckeite and graphene on MoS2. Thermoelectric properties of graphene nanoconstrictions are unveiled using a combined three-terminal approach. Additionally, cryoSThM is introduced as a new tool with the ability to measure at temperatures below 150K. The research presented in this thesis has a two-fold impact. On one hand, major technical SThM challenges are answered and efficient solutions developed. In this respect, cryoSThM and 3D nanothermal probing of materials open radically new routes for further investigations. On the other hand, new insights are gained from the extraction of materials properties and the observation of new phenomena. The knowledge gained through this research leads to innovative keys to develop applications in various fields, ranging from heat management to thermoelectric energy conversion

Quantitative Nanothermal Study of 2D materials by SThM and Finite Elements Simulations

The challenging measurements of thermal properties are of fundamental importance in the evolution of modern technology and direct quantification of nanoscale features is a crucial step in this development. Exploring confined systems from monolayer to bulk, we performed scanning thermal microscopy (SThM) on graphene, MoS2 and Bi2Se3 and correlated the outcomes with finite elements (FE) simulations. SThM is a scanning probe microscopy technique derived from the well-known Atomic Force Microscope where a self-heated probe is used as a thermosensor while scanning the sample; during probe-sample contact the corresponding drop in probe temperature can be electronically monitored and directly related to changes in the thermal properties as represented on Fig. 1 where a 5±1 nm flake of each materials mentioned on a SiO2 substrate is thermally measured. FE simulations were realized to correlate the measured properties of our systems varying several parameters such as graphene’s isotropy and anisoptropy as well as substrate interactions. We have investigated how these properties change as a function of sample number of layers on substrates of both high and low thermal conductivities. We observe well defined values of thermal conductance for monolayer and near monolayer thicknesses, however when increasing multilayers most materials conductance does not scale linearly with thickness as in some cases the conductance increases whilst in others a decrease is observed. We discuss and compare experimental considerations and simulation outputs in order to construct a thermal conductance models to explain these interesting results

Probing thermal transport and layering in disk media using scanning thermal microscopy

With the advent of heat assisted magnetic recording (HAMR) [1] the thermal transport properties of magnetic recording media have become a key performance characteristic. In particular it is important that lateral heat transport is minimised in order to heat only the localised bit area and conversely that vertical heat transport is optimised for fast cooling of the medium essential for the thermal stability of written bits. Magnetic media are multilayered and highly structured on the nanoscale rendering classical treatment of thermal transport inapplicable and the likelihood that the transport is dominated by interfaces and dimensions rather than bulk material properties. A technique for measuring thermal transport on the nanoscale is therefore highly desirable in the design of new magnetic media. In this study we explore the potential of scanning thermal microscopy (SThM) to resolve thermal transport on the nanoscale and use a multilayered, grain segregated conventional disk

Probing thermal transport and layering in disk media using scanning thermal microscopy

With the advent of heat assisted magnetic recording (HAMR) [1] the thermal transport properties of magnetic recording media have become a key performance characteristic. In particular it is important that lateral heat transport is minimised in order to heat only the localised bit area and conversely that vertical heat transport is optimised for fast cooling of the medium essential for the thermal stability of written bits. Magnetic media are multilayered and highly structured on the nanoscale rendering classical treatment of thermal transport inapplicable and the likelihood that the transport is dominated by interfaces and dimensions rather than bulk material properties. A technique for measuring thermal transport on the nanoscale is therefore highly desirable in the design of new magnetic media. In this study we explore the potential of scanning thermal microscopy (SThM) to resolve thermal transport on the nanoscale and use a multilayered, grain segregated conventional disk

Nano-mapping of Surface and Subsurface Physical Properties of 2D materials

A massive interest in two-dimensional materials (2DM) triggered by graphene (GR) discovery1 is fueled by the unique electronic, mechanical and thermal properties of these few-atomic-layers-thick materials. While electronic properties of graphene and other 2DM’s such as MoS2, WS, Bi2Se3, were extensively studied, their mechanical and thermal properties, equally record-breaking, are much less explored, due to inadequate tools for nanoscale probing of physical properties of atomically thin layers. Here we overcome this by combining atomic force microscopy (AFM) with specialist nanomechanical, nanothermal and nanoelectrical probes. By applying these to the single and few layer Gr and MoS2 we were able to explore the nanomechanical interaction of 2DM’s and the substrate, including layers adhesion and stresses; observe internal defects in the few layer 2DM’s, and defect movement under applied strain; map the nanoscale distribution, and quantify electrical charges trapped at the 2DM-substrate interface; observe with microscale and nanoscale resolution local electrical and thermal transport in these materials

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

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

Correlation of nanoscale electromechanical and mechanical properties of twisted double bi-layer graphene via UFM, PFM, and E-HFM

Recently, multiple theoretical and experimental studies have been published regarding the properties of stacked two-dimensional (2D) layers forming a twisted heterostructure. This field (known as twistronics) shows that properties of 2D materials can be modified to a great degree, including bandgap modulation and creating superconductive structures. Given the versatility that these structures have, many exciting engineering is being applied to them resulting in promising properties. In this study, we investigated a heterostructure composed of two twisted graphene bi-layers with a small angle between them (1.1º), where an atomic reconstruction is induced changing the lattice symmetry and creating a Moiré pattern. The electrical and mechanical properties of the 2D nanostructure are affected by this symmetry reconstruction, generating relaxation-induced strain gradients. We compared nanomechanical mapping via Ultrasonic Force Microscopy (UFM) and electromechanical response probed by Piezoresponse Force Microscopy (PFM) and Electrical Heterodyne Force Microscopy (E-HFM). These allowed us to assign Moiré patterns of the heterostructure to the particular crystallographic arrangements and to quantify the local Young’s modulus variation between single and double domain walls. Moreover, by measuring these domain walls specifically with PFM, it is possible to extract evidence of non-uniform strain in stretched triangular domains in the Moiré pattern. The phase images from the E-HFM allow us to observe a fast time-domain nanoelectromechanical relaxation in the order of picoseconds with nanoscale lateral resolution

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]

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