Three-Dimensional Nanothermistors for Thermal Probing | ACS

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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 22655−22667

Three-Dimensional Nanothermistors for Thermal Probing Jürgen Sattelkow,†,∇ Johannes E. Fröch,§,∥,∇,¶ Robert Winkler,† Stefan Hummel,⊥ Christian Schwalb,# and Harald Plank*,†,‡,§

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†

Christian Doppler Laboratory DEFINE and ‡Institute of Electron Microscopy and Nanoanalysis, Graz University of Technology, 8010 Graz, Austria § Graz Centre for Electron Microscopy, 8010 Graz, Austria ∥ Institute of Biomedical Materials and Devices, University of Technology Sydney, Ultimo, New South Wales 2007, Australia ⊥ Physics of Nanostructured Materials, University of Vienna, 1090 Vienna, Austria # GETec Microscopy Inc., 1120 Wien, Austria S Supporting Information *

ABSTRACT: Accessing the thermal properties of materials or even full devices is a highly relevant topic in research and development. Along with the ongoing trend toward smaller feature sizes, the demands on appropriate instrumentation to access surface temperatures with high thermal and lateral resolution also increase. Scanning thermal microscopy is one of the most powerful technologies to fulfill this task down to the sub-100 nm regime, which, however, strongly depends on the nanoprobe design. In this study, we introduce a three-dimensional (3D) nanoprobe concept, which acts as a nanothermistor to access surface temperatures. Fabrication of nanobridges is done via 3D nanoprinting using focused electron beams, which allows direct-write fabrication on prestructured, selfsensing cantilever. As individual branch dimensions are in the sub-100 nm regime, mechanical stability is first studied by a combined approach of finite-element simulation and scanning electron microscopy-assisted in situ atomic force microscopy (AFM) measurements. After deriving the design rules for mechanically stable 3D nanobridges with vertical stiffness up to 50 N m−1, a material tuning approach is introduced to increase mechanical wear resistance at the tip apex for high-quality AFM imaging at high scan speeds. Finally, we demonstrate the electrical response in dependence of temperature and find a negative temperature coefficient of −(0.75 ± 0.2) 10−3 K−1 and sensing rates of 30 ± 1 ms K−1 at noise levels of ±0.5 K, which underlines the potential of our concept. KEYWORDS: 3D nanoprinting, additive direct-write manufacturing, focused electron-beam-induced deposition, scanning thermal microscopy, nanomechanics, nanoelectrics, nanothermics coefficient determination,9 and heating of plasmonic structures,10 just to name a few. The success of such studies, however, is strongly related to suitable nanoprobes with respect to their dimensions, sensitivities, and stabilities.3,7 Commercial SThM tips can basically be divided into three different types. Wollaston probes use bridged metal wires (often Pt-based) attached to a prestructured cantilever basis.1,6,9 Although tip radii of down to 20 nm7,11,12 have been reported using very special treatments, typical apex radii are in the range of some hundreds of nanometers to a few micrometers,3,13,14 which are strongly limiting achievable resolution and positioning accuracy. The second approach uses U-shaped cantilevers, consisting of highly doped Si side-wall elements and low-doped tip areas in between.1,15 Although tip radii below 100 nm can be achieved,16,17 the “active areas” are fully connected with the

1. INTRODUCTION In research and development, thermal influences play a central role as physical, chemical, and functional properties often rely on local temperatures, its spatial distribution, and the temporal evolution in three-dimensional (3D) space.1,2 Accessing these details quantitatively is comparably easy at macroscopic dimensions, gets more complicated at the microrange, and becomes very challenging when entering the nanoscale. The latter, however, becomes increasingly relevant due to the still ongoing trend to smaller structures.3 Therefore, to specifically improve material design, the accessibility of thermal properties on the lowest nanoscale is not only important but often decisive for further optimization of thermoelectric nanomaterials.4 Among the comparably small pool of techniques, which allow thermal probing in the sub-100 nm range, scanning thermal microscopy (SThM) is the most powerful technology to access laterally resolved temperatures and further correlate the findings with surface morphology.1,5−7 Impressive examples have been demonstrated, such as Joule self-heating of graphene,8 Seebeck © 2019 American Chemical Society

Received: March 15, 2019 Accepted: June 3, 2019 Published: June 3, 2019 22655

DOI: 10.1021/acsami.9b04497 ACS Appl. Mater. Interfaces 2019, 11, 22655−22667

Research Article

ACS Applied Materials & Interfaces entire cantilever, representing a huge heat sink. By that, low temperatures are more complicated to access when operated in SThM mode.1,5,18 A combination of both approaches led to the introduction of surface-modified cantilevers: lithography-based methods are used to selectively fabricate metal structures across the tip, which can be understood as structured, conductive atomic force microscopy (C-AFM) tips. This approach enables small “active areas” down to 20 nm gap widths, however with end radii in the range of about 50 nm.1,5,19−21 More advanced approaches use carbon nanotubes,11 single-dot-assisted regions,22 or direct current imaging through the samples using the contact potential as “sensing element” (restricted to conductive samples).23 In this study, we introduce a 3D nanoprobe (3DNP) concept, which combines several aforementioned advantages in a single probe. In brief, a freestanding, multilegged, 3D bridge architecture is used, which is electrically operated to access temperature-dependent resistance changes (thermistor)24 once in contact with the sample surface. Fabrication is done via focused electron-beam-induced deposition (FEBID), which has matured in recent years.25 Aside of strongly improved predictability/reliability,26,27 FEBID was also used for 3D nanoapplications, including plasmonics,28 magnetics,29,30 and sensors31 as it enables direct-write fabrication of complex, freestanding 3D nanostructures27,32 with branch diameters below 100 nm and tip end radii in the range of 10 nm without any further processing. By that, such freestanding 3DNPs provide very small active volumes in contrast to the large heat sinks for planar thermal nanoprobes, which is a decisive element for fast and sensitive temperature sensing. On the other hand, the sharp apex is essential when aiming for the highest possible lateral resolution, which exploits the full potential when applied in vacuum conditions due to the absence of any resolution diminishing, convection-based heat transfer.3 Aside from the 3D capability, FEBID allows direct-write fabrication on practically any given material and surface morphology without specific preor post-treatment steps at the region of interest. Hence, this nanoprinting technique is ideal for fabrication and/or modification of already finished devices such as AFM selfsensing cantilevers (SS-CLs), as relevant in this study, which eliminates more complicated fabrication procedures during functional probe formation. While the long-term goal of our activities is the application of FEBID-based 3DNPs for high-resolution thermal mapping via SThM in vacuum conditions, this first study exclusively focuses on the detailed characterization of the actual 3D nanothermistor elements. This starts with the 3D design, nanofabrication, and optimization, goes over property tuning for stable AFM operation, and ends with the temperature-dependent, electric response in static and dynamic conditions as originally intended. By that, this study delivers the proof of principle for the application of FEBID-based 3D nanostructures as thermal nanothermistors for local temperature probing and lies the foundation for the aforementioned SThM-related follow-up activities.

Figure 1. Conceptual route toward 3D thermal nanoprobes. The basis are self-sensing cantilevers (gray), which are prestructured with Au electrodes (yellow). The actual 3D nanoprobe (3DNP, bright blue) is placed on top of a truncated tip region and electrically bridges two electrodes. Thermal probing is based on temperature-dependent resistance changes through the 3DNP (thermistor), once in contact with the surface of interest. The modular study starts with the overall 3DNP design (step 1) and then focuses on material modification to enable stable AFM operation (step 2). Finally, the electric response of the 3DNP thermistor is studied in thermally static and dynamic conditions (step 3) using a variable heating stage (red dashed circle).

(SEM)-assisted in situ AFM experiments for quantitative stiffness validation. In a second step, we focus on imaging quality of such 3DNPs during AFM operation and introduce a material tuning approach to increase the mechanical wear resistance. In the final step, we study the temperature-dependent electric response in static and dynamic conditions to deliver the intended proof of principle of our 3DNP concept for thermal nanoprobing. 2.1. Simulation-Based Design Selection. The first focus lies on the mechanical properties of freestanding 3DNPs in dependence on overall architectures and related dimensions. This is of essential relevance due to the unavoidable vertical and lateral force load during AFM operation. Therefore, both vertical and lateral stiffnesses (kV and kL, respectively) have to be maximized by proper design, which is studied in this section. Instead of an exhaustive trial-and-error approach between 3DNP fabrication via FEBID and mechanical characterization using AFM, FES was used to derive design-to-stiffness relationships for the upfront confinement of most promising architectures and related dimensions. The therefore required Young modulus has recently been extracted by Arnold et al.,31,33 who found a value of 14 ± 2 GPa for freestanding FEBID nanopillars. The application of this value as FES input parameter is justified as 3D structures in the mentioned study as well as in the present study were fabricated in the same dual-beam microscope (DBM) using the same FEBID parameter (30 keV, 21 pA), precursor (MeCpPt(IV)Me3), and technical setups. In this first step, stationary modeling was performed in COMSOLs Structural Mechanics Module. As no information on the Poisson ratio ν for nanocrystalline Pt in a disordered hydrocarbon matrix is available in the literature, we conducted an upfront study to evaluate its influence on final stiffness. For that, we compared simulated stiffness for a 5 μm high and 50 nm wide nanopillar with analytical solutions, both for increasing Poisson ratios from 0 to 0.5. For ν close to 0.5, the biggest deviations were 0.1 and 1% in vertical and lateral directions, respectively (see Section 1, Supporting Information). This

2. RESULTS AND DISCUSSION For the proof of principle concerning the application of freestanding 3D multipods as AFM-compatible, thermal nanoprobes, the study is split into three parts, as schematically summarized in Figure 1. First, finite-element simulations (FESs) are used for upfront confinement of 3DNP architectures and dimensions, complemented by scanning electron microscopy 22656


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Figure 2. Mechanical stiffness scaling and local stress distribution. FES-based scaling of mechanical stiffness during vertical (a) and lateral (b) force loads for a single pillar in dependency on its diameter and height (note the very different stiffness scales). (c−e) Qualitative, local stress distribution under vertical (top row) and lateral (bottom row) force loads for a single pillar (c), a bipod (d), and a tetrapod (e). The inclination angle α and the different force directions with respect to the bipod plane (kL⊥ and kL∥) are indicated in the top and bottom parts of (d), respectively.

overall negligible influence of ν can be rationalized by the high aspect ratio of the geometry, which implies that an axial volume change will result in an almost negligible lateral volume variation. For further simulations, ν was therefore fixed to 0.2, in agreement with literature values for carbon-based materials.34,35 Next, kV and kL were calculated by k = F/Δl with the applied force F and the obtained compression Δl for different pillar lengths (1−5 μm) and diameters (20−100 nm). The results are shown by 3D plots in Figure 2a (kV) and Figure 2b (kL), revealing stiffer behavior for larger diameters (linear scaling with cross-sectional areas) and shorter overall heights (linear scaling with reciprocal height; both effects are shown in Section 1, Supporting Information). More importantly, however, is the finding that kV exceeds kL by typically 4 orders of magnitude (see stiffness scales in Figure 2). For example, FES predicts vertical and radial stiffnesses of 13 and 10−3 N m−1, respectively, for a 3 μm high and 60 nm wide nanopillar. To evaluate FES reliability, we compared the results with the analytical solution36 using ΔlV = ΔlL =

range (see Section 1, Supporting Information), confirming the reliability of our FES approach. Figure 2c shows a 60 nm wide and 3 μm high Pt−C single pillar under axial (top) and radial (bottom) force of 10 nN together with the spatial stress distribution (see qualitative color bar). While axial compression leads to spatially homogeneous stress, radial force loads generate highest stress at the fixation point at the bottom, which becomes relevant later. In the second step, we changed the design to bi-, tri-, and tetrapods (TP) with a constant pillar diameter of 60 nm and simulated kV and kL in dependency on total heights h and inclination angles α (measured against the substrate surface). Figure 2 shows the stress distribution in bi- (d) and tetrapods (e) under vertical (upper) and lateral (lower) force loads (tripods are shown in Section 1, Supporting Information). In contrast to single pillars (c), multipods reveal the highest stress at the topmost merging area under vertical force load (compare upper models in (c)−(e)), which indicates that the tip apex will experience the highest stress during vertical force load in AFM. Besides these qualitative findings, Figure 3a presents quantitative kV scaling plots for all architectures (abscissa) and selected variations in h, α, and d. As reference, multipod heights of 1 μm, inclination angles of 60°, and branch diameters of 40 nm are shown by black squares. The red circles reveal implications of structure heights (1 → 5 μm), the blue triangles indicate steeper angles (60 → 80°), and the purple inverted triangles show the effect of broader branches (40 → 80 nm). The

4FVh πd 2E

(1)

64FLh3 3πd 4E

(2)

with h as pillar height, d as diameter, and E as Young’s modulus. The results revealed a deviation of less than 1% over the studied 22657


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character for larger inclination angles is also in agreement with single-pillar findings as increasing α (towards 90°) can be understood as a convergence to vertical pillars with highest possible stiffness. The radial stiffness behavior, shown in Figure 3b, is more complex due to the dependency on force load directions relative to multipod orientation. This becomes most pronounced for bipods, when comparing force loads perpendicular (kL⊥) and parallel (kL∥) to the bipod plane (see indications in the lower part of Figure 2b), resulting in a strong stiffness splitting (duplets in Figure 3b, connected by solid/dashed lines). While the former is identical to lateral flexing of single pillars (see bottom models in Figure 2c,d), the latter resembles an on-axis compression. This leads to kL⊥ and kL∥ values that differ by more than 3 orders of magnitude, which immediately points out the need for a radially resolved kL study. The red curve in Figure 3c shows the result for a bipod structure, which actually is a very small ellipse. This finding immediately excludes bipods as potential AFM probes, as stable XY scanning becomes practically impossible. While the radial kL homogeneity strongly increases for tripods (blue curve in Figure 3c), the situation becomes very homogeneous with a four-legged tetrapod (TP) architecture, shown by the almost circular green curve in Figure 3c. Radial stress distributes homogeneously across TPs, as shown in the lower part of Figure 2e, thus making it the most favorable architecture. Analogous to kV, Figure 3b shows quantitative kL scaling plots for the same geometrical variations. As mentioned before, the bipod splitting stems from the kL⊥/kL∥ anisotropy, where higher values are always connected to force loads parallel to the bipod plane. While increasing heights and diameters lead to the same qualitative shifts for kL as for kV, the dependency on inclination angles α is inverted: high angles are beneficial for vertical stiffness, while dramatically reducing its lateral counterpart. This implies that a compromise has to be found for the inclination angles to provide the highest kV with acceptable kL values. The green diamonds in Figure 3a,b show such a compromise taking realistic pillar diameters, fabrication capabilities (topmost merging volume), and geometrical boundary conditions on self-sensing cantilever (minimum footprint) into account as well. In detail, TP geometries with 60 nm branch diameters, 2 μm overall heights, and 70° inclination angles were chosen for further experiments due to radially homogeneous behavior and comparably high kV and kL values of 60 and 4 N m−1, respectively. The former value exceeds the target stiffness of the cantilevers by 1−2 orders of magnitude (nominally 4 and 8 N m−1 for two different cantilever types). In the next step, real experiments are presented for quantitative validation of our FES studies. 2.2. Experimental in Situ Characterization. To evaluate FES results, real compression experiments were performed in a scanning electron microscope-integrated in situ AFM37 instrument (AFSEM by GETec Microscopy Inc.) to study the morphological behavior during force load. For that, self-sensing cantilevers were prepared via focused ion beam (FIB) processing by removing the front part for real-time SEM inspection (see Section 2, Supporting Information). The apex of the original tip has been removed with a pretilt to produce a parallel contact area between AFM probe and sample surfaces, as shown in Figure 4a by an SEM side view image. Next, TP arrays were fabricated on Si−SiO2 substrates, using the simulated design to evaluate the scaling behavior for comparison to FES. First compression tests, however, revealed an unexpected

Figure 3. Architecture-dependent stiffness scaling and radial homogeneity. (a) Vertical stiffness (kV) vs the architecture type for selected heights, diameters, and inclination angles (indicated in Figure 2d) to see individual influences. Of particular relevance is the final design shown by green diamonds as described in the main text. (b) The same plot as shown in (a), however for radial stiffness (kL). For bipods, there are always two points that represent force loads perpendicular (kL⊥) and parallel (kL∥) to the bipod plane (see Figure 2d), corresponding to the lower and higher values, respectively. (c) Normalized, lateral stiffness in dependency on the force load direction. For the bipod (red ellipse), the force directions are indicated in agreement with indications in Figure 2d. While the tetrapod (green) is circular, the tripod shows slight deviations of less than 4 rel.% according to its threefold symmetry (dotted blue).

first detail is the linear stiffness increase with the number of branches, except for single pillars, which are always vertical (α = 90°). In agreement with single-pillar results (Figure 2a), taller structures lead to decreased stiffness (red arrow), while thicker branches increase the values for kV (purple arrow). The stiffer 22658


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Figure 5. Identification of twisting due to spatial fabrication mismatch. SEM inspection of tetrapods in a tilted (a) and top-view arrangement (b), which both reveal spatial inaccuracies in the central merging region. To study the implications of this mismatch, FES experiments were conducted (c), which use different types of misalignments (see inset in (c)). This approach could well mimic the observed twisting effects during real experiments (d). (d) Irreversibly deformed tetrapod from top after multicycle compression, which is qualitatively identical to dynamic deformation during force load (c).

Figure 4. Tetrapod twisting during force load. SEM-assisted AFM compression tests used FIB preprocessed cantilever to generate a parallel plane with respect to the substrate (a). (b−d) Compression series of a tetrapod starting with the approach (b) over the compression (c) and further release (d) of the force load (see also Movie 1, Supporting Information). In this case, both the strong twisting during compression and slight irreversible deformation after compression are observed (see red arrows and also Figure 5).

along with another morphological adaption. For completeness, we want to mention that a nonvertical compression (lateral flexing) due to experimental inaccuracies was considered via FES as well, which, however, could not reproduce the observed twisting effects (see Section 2, Supporting Information). During the same experimental series, nontwisting TPs were furthermore subjected to AFM-based quantitative mechanical characterization. For this, the preprocessed cantilever were first calibrated against Si−SiO2 surfaces to recalculate the cantilever stiffness kCL. Next, ramp cycles with limited Z movement were performed on tetrapods, while monitoring the exerted force. Figure 6a shows a set of response curves using a cantilever with a stiffness of 7.2 ± 0.2 N m−1. The steep, black dotted line gives the reference response on Si−SiO2. The colored solid lines give the raw data of a multicycle response for the same TP (see legend), which immediately reveals the astonishing result of a saturation behavior. As evident, a widely linear range is observed for small displacements (shaded green), followed by a weak kink (blue arrow) and another widely linear regime (shaded blue), which eventually changes into a strong saturation behavior (shaded red). This means that at a certain point (blue → red), the required force to further lower the cantilever strongly decreases and saturates in almost zero efforts for further compression (red region). This remarkable result stems from the cycles actually lying on top of each other, clearly indicating a reversible compression even after 10 cycles. While the example in Figure 6a reveals a practically identical onset, offset behavior was found as well for some tetrapods, which indicates

peculiarity by means of a twisting behavior. A particularly clear example is shown in Figure 4, starting with the first AFM approach (b) followed by a vertical compression of 350 nm (c) and after force release (d) (see also Movie 1, Supporting Information). Although small in this example, the TP suffered an irreversible damage as indicated by the red arrows in (d) (compared to (a)). During a large series of compression experiments, many different deformation types during and after compression were found (examples can be found in Section 2, Supporting Information). As such twisting behavior was not predicted by FES, we subjected the TPs to a closer SEM inspection, which revealed spatial mismatches in the topmost merging zones. Figure 5 shows an example by means of a vertical displacement (a) and an even stronger lateral mismatch in the central merging zone (b). When including real mismatch values derived from SEM in the FES model (inset in c), FES was able to mimic the deformation mode as found by experiments as shown in Figure 5d. Note that the latter SEM image was taken after compression and actually shows an irreversible deformation, which is identical to the bending direction during dynamic compression observed by live SEM imaging. These findings clearly point out that the highest spatial precision is indispensably required to avoid uncontrolled deformation. The solution for this problem is a slightly adapted design in the merging region, as described later in more detail as it goes 22659


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TP arrays with varying heights and target angles were fabricated, initially characterized via SEM to access real heights, αeff and αTO values. The same TPs were then subjected to quantitative AFM compression tests, where only nontwisting TPs were used for analysis. The results can be seen in Figure 7 by a direct quantitative comparison between experiments (a) and simulations (b). As evident, a very good agreement with relative

Figure 6. Identification of nonlinear compression effects due to nonstraight branches. (a) AFM-based force vs displacement curves with the reference curve of the cantilever against the substrate by the dotted, black line. All curves are unprocessed raw data. The solid curves illustrate compression behavior on tetrapods revealing two widely linear (green and blue) regimes, followed by a strong saturation behavior (red). Recalculation of the vertical tetrapod stiffness suggests 18 ± 2 N m−1 following the indicated equation. (b) Tilted SEM image of a tetrapod, which reveals the nonstraight character of the side branches (see αeff). The red dotted line indicates the takeoff angle αTO, which was used for calculating the angle deviation Δα (see definition on top). (c) These data were then used for FES accessing the vertical stiffness in dependency on Δα, revealing a fast drop by a factor of 4 for Δα ∼ 5°. The top insets show the according models, which reflect the nonstraight side branches.

irreversible, plastic deformation after compression cycles, excluded from these analyses. When considering the TP with a collective single-spring cTP, equivalent to kV, one can use the formalism for serial springs to recalculate this value from the first linear region (shaded green), suggesting a cTP of 18 ± 2 N m−1 in this case. While FES does not predict the saturating behavior, a closer look reveals another morphological deviation by means of nonstraight side branches of the TP, as shown by a tilted SEM image in Figure 6b. The deviation from straight substrate-tip geometries (dashed yellow lines) leads to a special form of Euler buckling with both ends fixed38 although a certain degree of free translation parallel to the substrate can occur. To simulate such a behavior, we first introduce the takeoff angle αTO together with the intended angle αeff connecting start- and end-point of the side branches, both shown in Figure 6b. Next, we modeled the branch bending by a second-order polynomial, which fits well real circumstances (see Section 2, Supporting Information). The results of the adapted FES are summarized in Figure 6c, showing the influence for increasing Δα (defined by αTO − αeff) on vertical stiffness (ordinate). Starting with a cV of 20 N m−1 for perfectly straight side branches (green indications) without any spatial mismatch at the apex, even a small deviation of only 5° leads to a stiffness drop by a factor around 4 (indicated in blue). This fast decay can be understood as a transition from axial compression along the branches to lateral flexing for increasing bending angles, which clearly points out the high relevance for straight side branches. To evaluate the validity of the FES results,

Figure 7. Design adaption and FES validation. (a) Vertical stiffness simulation using SEM-based values for inclination angles, heights, and deviations from straight branches whenever found. (b) The same plot but with real experiments using the same tetrapods, which reveals a good qualitative and quantitative agreement with vertical stiffness values up to 70 N m−1. Experimental errors are ±2 N m−1. (c) SEM topview image of an optimized tetrapod, where the slightly increased merging region by the introduction of additional pattern points can be seen (blue arrows). This becomes also evident in a SEM side view in the left image of (d), which shows a tetrapod before AFM compression. Furthermore, the process adaption by additional refresh times during 3D growth leads to very straight side branches, also evident in (d). The right-hand side of (d) shows the same tetrapod after multicompression cycles, which leads to slight sideward bending, while the overall geometry is still maintained (see also Movie 2, Supporting Information). 22660


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2.4. Property Tuning. As base area for the optimized TPs, the original tip of a self-sensing cantilever was FIB-processed, resulting in a flat top plateau with 800 nm side length. Next, FEBID was used for TP fabrication, as shown in Figure 8a by a

deviations of less than 10% was found, which clearly reveals the nonlinear force-to-compression behavior as a consequence of nonstraight branches. Finally, we simulated the dynamic response during force load by including the cantilever in the simulation as well, which showed a good agreement with real experiments as well (see Section 2, Supporting Information). This validates our FES approach, including the initially used value of Young’s modulus, to predict spatial stiffness for FEBIDbased multipod architectures, and explains undesirable artifacts such as radial twisting and nonlinear behavior that occur during AFM compression experiments. 2.3. Design Optimization. While ideal architectures (fourlegged TPs), including relevant overall heights and inclination angles, can be derived from FES, further design optimization is needed to minimize twisting and nonlinearity effects. As described before, the former occurs due to lacking precision in the merging apex region, which typically stems from mechanical stage drift and nonideal boundary conditions during FEBID fabrication, both discussed by Winkler et al.25 Although these effects can technically be minimized, it is difficult to achieve reliable accuracy in the lowest nanometer range in 3D space. To overcome this challenge, we have increased the volume in the merging region, making the apex more rigid, thus less sensitive to very small deviations and less prone to twisting effects. Figure 7 shows SEM top view (c) and tilted images (d), where the slightly increased merging region is evident, realized by additional patterning points. The second unwanted effect, namely, the nonlinear response under force load, is a consequence of nonstraight branches. As recently revealed, this artifact has its origin mainly in local heating during FEBID.39 As branch growth proceeds, the temperature at the beam impact region increases, resulting in a reduction of precursor coverage and therefore reduced growth rates. While optimization toward minimized bending was done by the introduction of additional refresh times in this study, we are currently working on the integration of a temperature compensation module in the CAD software 3BID.26 Figure 7d shows the results of this adapted process strategy, leading to straight single branches. When compressing such idealized geometries, both twisting and irreversible damages are eliminated and minimized, respectively (see Movie 2, Supporting Information). Figure 7d shows a direct comparison of a TP before (left) and after multicycle compression (right). Prior to a final design selection, the boundary conditions for TP footprints had to be determined. This follows the electrode layout on our self-sensing cantilever, which currently requires a leg-to-leg distance of at least 500 nm. From the stiffness plots in Figure 7b, it becomes evident that shortest TPs with steepest sidewalls are most promising. Now choosing a realistic side-wall angle around 70° together with the aforementioned demand on the base width, TP heights between 700 and 1100 nm should lead to vertical and radial stiffnesses higher than 50 and 5 N m−1, respectively. Those values are 1−2 orders of magnitude higher than the planned cantilevers spring constants, and therefore should allow stable AFM operation as discussed in the following section. In short summary, a four-legged tetrapod geometry is chosen as it provides radially homogeneous stiffness, while the opening angle is dictated by the targeted axial stiffness and the applied footprint. Although higher leg numbers would further increase mechanical stiffnesses, this approach was not followed as it would also increase the active volume, which can impact both response times and temperature sensitivity.

Figure 8. AFM operation and wear effects. (a) Tilted SEM image of an FIB-truncated AFM tip (yellow arrow) for further fabrication of a 3D tetrapod as 3D nanoprobe (blue arrow). Such tips were then used in AFM contact mode on an FIB-processed test structure. (b) Representative height image, which was taken with 250 nN force load and a scan speed of 20 μm s−1 (see also Movie 3, Supporting Information). This image shows raw data, except for a first-order plane tilt. The poor image quality could be traced back to a wear effect of the tip apex region, shown by a direct comparison before and after AFM operation in (c) and (d), respectively.

tilted SEM image. Such 3D nanoprobes (3DNP) were then used for AFM measurements on an FIB-processed test layout. Figure 8b gives a representative 3D height image, obtained in contact mode with a constant force load of 250 nN and a scan speed of 20 μm s−1. Dedicated scan speed tests revealed stable operation up to 200 μm s−1 (see also Movie 3 and Section 3, Supporting Information). While these experiments confirm the general applicability of 3DNPs as AFM tips, image quality is evidently poor by means of high noise, line jumps, and blurred edge features (Figure 8b), which continuously worsened during scanning for more than 1 h. Figure 8 shows tilted SEM images before (c) and after AFM operation (d) of another 3DNP, which reveals the decaying image quality as mechanical wear effect at the tip apex. This finding is in agreement with the FES results of the highest local stress at the tip apex for multileg architectures (see Figure 2e). The origin of this low wear resistance can be attributed to the inner structure of FEBID-based materials, which consist of nanosized platinum grains (typical 2−4 nm in diameter) embedded in a carbon matrix with typical C contents around 85 atom %.40,41 The latter mainly stems from incompletely dissociated precursor molecules and nonvolatile fragments during FEBID and is unavoidable for this precursor material.40,42 22661


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Figure 9. Material tuning and AFM performance. (a) Tilted SEM images of the same 3D nanoprobe after fabrication (left), after electron-beam curing (middle), and after AFM operation (right). As evident, electron-beam curing (EBC) leads to a small, vertical shrinkage and a slight increase of the tip apex radius. After contact mode AFM for more than 4 h at a scan speeds up to 40 μm s−1 and a constant force load of 250 nN, the 3D nanoprobe maintains its overall shape and reveals only minor increase of the apex radius. (b) AFM height image, taken with the probe shown in (a) using 20 μm s−1 and a force load of 250 nN. The comparison to Figure 8b clearly reveals the impact of EBC by the strongly improved image quality. (c) Speed test series from 20 to 160 μm s−1 and back to 20 μm s−1 (bottom right). As is evident, the image quality starts to decrease at 80 μm s−1 but can be restored to the original quality when going back to 20 μm s−1, which indicates high wear resistance in agreement with the right SEM image in (a). AFM image processing was limited to a first-order plane tilt.

was incrementally increased to 160 μm s−1 with 20 min scan periods, each summarized by Figure 9c. As can be seen, the image quality slightly deteriorates at scan speeds higher than 80 μm s−1, but can be restored when going back to 20 μm s−1 (bottom right). Using the former scan speed with a resolution of 512 × 512 pixels, a 4 × 4 μm2 scan takes only 50 s. This is the same time scale as the acquisition of high-resolution SEM images, hence demonstrating the high-speed capability of our 3D nanoprobes. In the following, we subjected such EBCtreated 3DNPs to long-time measurements by scanning a total distance of about 55 cm in less than 4 h at a constant force load of 250 nN and a scan speed of 40 μm s−1. Although the image quality was still good (see Section 3, Supporting Information), we could recognize a slight feature broadening. This tip quality loss is not unusual for such operating conditions and total scan lengths. The right image in Figure 9a shows the same 3DNP after both scan speed variation (Figure 9c) and long-time scanning (Figure S20, Supporting Information), revealing a small increase of the tip apex radius (∼11 nm) in agreement with the aforementioned feature broadening. The main finding, however, is the absence of strong wear effects for EBC-treated 3DNPs even after long-time scanning (Figure 9a, right) compared to as-deposited 3DNP after just 1 h (Figure 8d). Hence, electron-beam curing has been proven as a successful material improvement approach to increase mechanical wear resistance for stable AFM operation as originally intended. 2.5. Thermal Response. The final goal of our approach is the fabrication of 3D nanoprobes on self-sensing cantilever (SSCL) for application with a novel in situ AFM platform, called AFSEM (GETec Microscopy Inc., Vienna, Austria). A central element of this system are piezoelectric self-sensing elements, integrated close to the cantilever−chip contact region, as shown in Figure 10a by a tilted SEM image. This approach enables

In a recent study, Arnold et al.31 focused on the mechanical properties of Pt−C nanopillars, demonstrated their tunability, and gave an explanation for its underlying reasons. In brief, Pt− C-based FEBID materials were subjected to postgrowth electron-beam curing (EBC),43 which entails two effects. First, incompletely dissociated precursor molecules are further fragmented, leading to a slight particle growth with strong implications on the electric properties, as initially demonstrated by the work group around Michael Huth in several studies.9,30,44−46 Complementary, Arnold et al. demonstrated a strong increase in the Young modulus during EBC by a factor of 5. The origin of this behavior was suggested to rely on the electron-beam-induced chemical modification of the carbon matrix from loosely bound sp2 networks over glassy carbon toward amorphous glass.47 Based on these findings, we applied the same procedure to our TPs with a typical curing dose of 150 nC μm−2 performed at 30 keV and 44 pA to minimize the risk of co-deposit by transmitting low-energy electrons and high beam currents.27 Figure 9a shows a series of SEM images of a 3DNP right after fabrication (left), after EBC with a dose of 150 nC μm−2 (center), and after long-time AFM scanning (right) as discussed in the following. As can be seen, the TP slightly shrank after EBC (∼30 nm) in agreement with the literature48 and revealed a small increase of the apex radius from ∼6 to ∼9 nm. Prior to AFM measurements, force-ramp experiments, as presented in Figure 6, were repeated, which indicated a linear behavior up to 250 nN with spring constants higher than 40 N m−1 (see Section 3, Supporting Information). Figure 9b shows a 3D AFM height image acquired with the 3DNP shown in the central SEM image (a) at identical AFM operating conditions used for the image in Figure 8b for as-deposited 3DNPs. As evident, the image quality has significantly improved and provides lateral resolution below 10 nm. Next, the scan speed 22662


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Figure 10 by SEM side- (c) and top-view (d) images. After fabrication, 3DNPs were subjected to postgrowth EBC to improve mechanical wear resistance as discussed in the previous section. As mentioned before, electron-beam curing also affects electrical properties as the slight grain growth reduces grain-tograin distances for the here applied doses, which increases the tunnel probability and thus the electric conductivity. As this process eventually stops, electric in situ measurements can be used to control the curing process for defined and reproducible electric properties of 3DNPs. For our architectures, we typically observed first saturation tendencies around 10 nC μm−2, followed by the second linear regime around 70 nC μm−2, indicating the emergence of the graphitization-dominated regime (see Section 4, Supporting Information).31 EBC was always stopped after a total dose of 150 nC μm−2, resulting in bridge resistances between 10 and 20 kΩ, depending on initial 3DNP dimensions and now denoted as 3D thermal nanoprobes (3DTNPs). Figure 10e gives an I/U curve of the shown 3DTNP with a linear behavior even for small voltages, as evident by the double logarithmic inset. Taking into account typical bridge resistances around 10 MΩ before EBC, this means an improvement by almost 3 orders of magnitude in good agreement with the literature.40,45,48 For completeness, we want to mention that the shown curves are uncorrected raw data with a serial pre-resistor (RPRE) of 1 MΩ. Next, the electric resistance response of 3DNPs is studied in dependency on their temperature. To provide a reliable experimental setup, calibrated microelectromechanical system (MEMS) heater chips (Wildfire S3 chips, DENSsolutions, Delft, the Netherlands) were used as substrate with an in-house-built DBM chip holder, which provides full compatibility with our in situ AFM equipment (see Section 4, Supporting Information). Experiments were conducted on silicon-nitride-coated chips to prevent electric contact to the underlying Pt heating elements. For heat response measurements, 3DTNP-modified SS-CL with spring constants of 4−8 N m−1 were used, electrically operated by a constant current source at 10 nA while the voltage drop was constantly measured. Note that current sweeps up to ±1 μA did not reveal deviations from a linear I/U behavior, which would indicate Joule self-heating. All experiments were performed in quasi-static conditions, realized by a scan range of 5 × 5 nm2, with a force load of 50 nN. Figure 11a shows the relative resistance variation ΔR/R0 (left ordinate) during a slow temperature profile up to 80 °C and back to room temperature (solid blue line, right ordinate). As reference, unprocessed Aucovered SS-CL were used, which revealed no resistance change as is evident by the red line and circles. In contrast, 3DTNPs showed a clear and reversible response to temperature variation, as illustrated by the green curve and triangles in Figure 11a. Note that the shown data are unprocessed, spline-less raw data with a symbol distance of 30 values. Apart from this general proof of principle, the decreasing resistance requires a closer look (see inverted left ordinate). In nanogranular FEBID materials, electric transport takes place in the correlated variable-rangehopping regime and do not behave like a classical metal. As shown by Huth and co-workers, electric conductivity increases with temperature as intergranular (co-)tunneling is thermally assisted.45 This leads to a negative temperature coefficient (NTC) compared to positive temperature coefficients for most metals. By that, the here found decreasing resistance with increasing temperatures is consistent with theory. This behavior also explains why a constant current approach was chosen: the dissipated power by the measurement circuit is given by UI.

Figure 10. Thermal nanoprobe fabrication and basic characterization. (a) Self-sensing cantilever including the two self-sensing elements and the prestructured Au electrodes. (b−d) SEM close ups of a 3D thermal nanoprobe, showing the FIB-based fabrication of a flat plateau and the electrode splitting together with the 3D tetrapod, which electrically bridges the electrodes; (c) and (d) are partly colored to indicate the different layers, while still providing the original SEM image at the left. (e) I vs U curves after EBC with a dose of 150 nC cm−2, revealing a linear behavior. The small inset is a double logarithmic plot to demonstrate the linearity for small voltages as well. Recalculation of the resistances gives ∼8.6 MΩ before and ∼10.8 kΩ bridge resistance, meaning a resistivity reduction by a factor of ∼800 after EBC. Note: the presented curves are raw data using a 1 MΩ serial pre-resistor.

electric readout of cantilever deflection and twisting, thus replacing space consuming optical laser detector systems.44 By that, the entire system is highly compact for seamless integration in space-confined SEM, FIB, or dual-beam microscopes (DBMs) to enable complementary in situ AFM characterization. The target concept for thermal probing using 3D nanoprobes is based on electric thermistors, which effectively change electric resistivity in response to their temperature, broadly used as a temperature sensor concept for macroscopic applications (e.g., Pt100 element) but also for SThM probes.3,49,50 Here, the boundary conditions are two electrodes, which are bridged by our 3DNP with two legs, each. Applying a constant current (explained in detail later) while measuring the voltage drop across the bridge gives information about resistivity changes and by that indirectly about the temperature of the 3DNP. As indicated in Figure 10a, the original tip is covered by a 100 nm thick, prestructured Au electrode. To modify the Au-covered tip for thermal nanoprobe fabrication, an FIB-based two-step procedure is applied, followed by the FEBID fabrication of 3DNPs. First, the tip region is removed to form a 700 nm wide plateau with a pretilt of 11° to establish parallel orientation to the substrate during AFM operation. Next, the Cr/Au electrode is split across the entire tip region by the formation of a 100 nm wide trench, as can be seen in Figure 10b−d by a tilted and recolored SEM image. Electrical in situ measurements were used to accurately stop FIB milling once the path is opened, indicated by a sudden resistance increase in the instrument compliance (R > 800 MΩ). Finally, 3DNPs were fabricated with 600 nm wide, squared footprints and inclination angles around 70°, resulting in heights around 1200 nm, with branch diameters around 85 nm. The 3DNPs were placed in such a way that two legs were connected at each electrode as mentioned above and shown in 22663


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to the literature reported, resistive SThM probes, the absolute NTC values of our 3DTNPs are also in the same order of magnitude,52 e.g., Wollaston bridges (1.6 × 10−3 K−1)53 or Pdcoated small-scale thermistors (3.6 × 10−3 K−1).54 Further comparison to other concepts is partly complicated as many approaches use, e.g., Wheatstone/Thomson bridges and/or alternating current (AC) readout approaches such as 3ω,3,50,53,55,56 which amplify the sensitivity in comparison to our approach. Wielgoszewski et al. demonstrated the smart adaption of readout electronics and achieved a sensitivity increase from 0.91 μV K−1 (Thomson) over 5.43 μV K−1 (Wheatstone) up to 7.05 μV K−1 (modified Wheatstone).55 Recalculating the values for our 3DTNPs gives intrinsic sensitivity values of 0.8−1.5 μV K−1 (depending on the final geometry) without any bridge electronics, which is very promising once configured in advanced readout electronics as planned for the future. As nanogranular FEBID materials are known to change their electric properties by compressive and tensile deformation,44,48 mechanical compression influences were tested as well. Gradual force increase up to 75 nN at different temperatures revealed voltage offsets of less than 1 rel. %, while NTC values at constant pressures remained the same with the error margin of 0.2 × 10−3 K−1 (see Section 4, Supporting Information). In the final step, we maximized the MEMS heating ramp to evaluate the achievable sensing rate for 3DTNPs. This is of particular relevance, as one of the main arguments for our 3D concept are the small active sensing volumes, which are a basic prerequisite for fast thermal response during both heating up and heat dissipation.3 Figure 11b shows the time-resolved response for 25−30 and 20−50 °C ramping profiles, again by unprocessed raw data. While the dotted blue line shows the temperature feedback from the MEMS heater (right ordinate), the green curve depicts again the relative resistance variation (left ordinate) acquired in 66 ms steps. As is evident, the slopes of both signals practically lie over each other, which reflects an immediate electric response during local temperature changes. Based on the slopes, a sensing rate of 30 ± 1 ms K−1 is found, while the temperature noise of ±0.5 °C in the plateau (see inset) is similar to the slow ramping experiments (compared to inset in Figure 11a). Note that even higher response rates for the 3DTNPs are very likely as both curves are very similar and the MEMS heater eventually limited the temperature ramping speed. Although faster MEMS heaters have to be applied in future to determine the achievable sensing rates, the presented data clearly confirm that our 3DTNP concept allows for fast, quantitative, and reversible temperature sensing, even without bridge electronics or more sophisticated AC readout approaches.

Figure 11. Thermal response in static and dynamic conditions. (a) Electric response of 3D thermal nanoprobes (3DTNP) for increasing temperatures performed on a heating MEMS chip. The red curve shows the response of an untreated, Au-covered self-sensing cantilever as reference together with the MEMS temperature readout given by the solid blue line (right ordinate). While Au is not changing the resistance (ΔR/R0, left ordinate), 3DTNPs reveal reduced resistances for higher temperatures due to thermally assisted tunneling transport in the nanogranular material. Note that the green curves are raw data with a symbol spacing of 30 points. The negative temperature coefficient of resistance is found to be −(7.5 ± 0.2) 10−3 K−1, while the noise level corresponds to ±0.5 °C (see raw data in the inset). (b) A similar plot to (a) but performed with the highest possible heating rates to evaluate 3DTNP sensing rates. Comparing the temperature readout of the MEMS (dotted blue, right ordinate) with the electric response signal for 3DTNPs (solid green raw data, left ordinate), the fast response character becomes evident. The response rate is estimated with 30 ± 0.2 ms K−1, while the temperature noise level is found to be similar to (a), as shown by the inset (ΔT ± 0.5 °C). All measurements were performed in quasi-static scanning conditions using a scan range of 5 × 5 nm2 with a constant force load of 50 nN.

Substituting U by IR gives P = I2R, which makes clear that decreasing resistances for higher temperatures lead to lower dissipation and by that prevents self-heating. In contrast, constant-voltage measurements would lead to P = I2/R with the consequence that decaying resistances increase power consumption, which can lead to unwanted self-heating. NTC values for the here studied 3DTNPs were found to be −(7.5 ± 0.2) × 10−3 K−1, which ultimately depends on volume and shape of the topmost merging region. A closer look on the noise level in the top plateau at 80 °C is given by the inset in Figure 11a by unprocessed raw data, revealing a recalculated temperature noise of less than ±0.5 °C derived from the maximum variation. The here extracted, absolute NTC value ranges in the same order of magnitude as for most metals (3.6 × 10−3−6.5 × 10−3 K−1) and/or alloys (0.2 × 10−3−6.3 × 10−3 K−1).51 Compared

3. CONCLUSIONS In this study, we have delivered the proof of principle for the application of FEBID-based 3D nanobridges as thermal nanoprobes with quantitative, reversible, and fast sensing properties, suitable for stable AFM operation. First, an FES model was developed and validated by experimental correlation, which identified the implications of lacking spatial precision by means of twisting, bending, and nonlinearity during force load. Optimization of both architecture and geometric dimensions led to 3D nanoprobe design with four legs (tetrapod) revealing vertical and radial stiffnesses higher than 50 and 5 N m−1, respectively. Furthermore, the highest local stress in 3DNPs during force load was predicted in the tip apex region, which was verified by AFM experiments leading to strong wear effects after 22664


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design within the software. While 30 keV and 21 pA were used for fabrication, 30 keV and 44 pA were used for postgrowth electron-beam curing in top-view arrangement. For compression experiments, 1 × 1 cm2 silicon substrates with 3 nm oxide layer were used.57 AFM imaging tests used the same substrates, which were sputter-coated with 100 nm Au and further FIB-processed at 30 kV and 10 pA using bitmap files. AFM tips were truncated via FIB, also using 30 kV and 10 pA. Before any SEM inspection, a gas clearance time of at least 1 h was introduced. Imaging parameters were 5 keV and 98 pA with the lowest possible dwell times to minimize unwanted effects. 4.3. AFM Characterization. 4.3.1. In Situ AFM. Compression and thermal studies used the AFSEM platform by GETec Microscopy Inc. (Vienna, Austria), which is designed for seamless integration in SEMs, FIBs, and dual-beam microscopes without limiting the main functionalities. A key feature are piezoelectric self-sensing cantilevers, which eliminate space consuming, optical detection systems. All experiments in this study were performed with cantilevers showing stiffness of 6−8 N m−1. Truncating of the original tip was performed via FIB as described in the previous chapter. The AFM is operated by a specially modified ANFATEC controller (Oelsnitz, Germany), which allows manipulation and extraction of electric signals toward the tip electrodes, as relevant for thermal measurements. Compression experiments used the classical ramp mode with controlled Z movement depending on the studied 3D multipods. Prior to any ramping experiment, the cantilevers were calibrated against the Si−SiO2 substrates to enable quantitative force measurements. Thermal response experiments were carried out in quasi-static scan situations using a 5 × 5 nm2 scan range and a force load of 50 nN. 4.3.2. Ex Situ AFM. Imaging experiments of 3D nanoprobes, including wear studies, were performed with a FastScan Bio System, operated by a Nanoscope V controller (Bruker Nano, Santa Barbara) using Tap150A cantilever (Bruker AFM Probes, Camarillo). The cantilever had a nominal spring constant of 5 N m−1 and were initially truncated via FIB for subsequent 3D nanoprobe fabrication, as described in a previous study. Prior to each experiment, the cantilevers were calibrated against Si−SiO2 substrates to allow for quantitative force measurements. All imaging experiments in this study were carried out in contact mode with a force load of 250 nN, while gain parameters were optimized for the highest quality possible. Analyses were done by Nanoscope Analysis 1.5 software (Bruker Nano Surface, Santa Barbara) or by Gwyddion v.2.51.58 If not stated otherwise, images were only subjected to a first-order plane tilt. 4.4. MEMS Heating. Thermal response measurements for 3D nanoprobes were performed on calibrated MEMS heater chips (Wildfire S3 chips, DENSsolutions, Delft, the Netherlands). To operate the heater chips in our NOVA 200 dual-beam microscope, a sample holder was designed, fabricated, and tested in house. Heating and temperature sensing was performed with the original equipment by DENS solution using the software Digiheater (V.3.2, DENS Solutions, Delft, the Netherlands), which allows controlled ramping, holding, and on-demand temperature profiles, while reading out actual temperatures. As mentioned before, 3D nanoprobes, fabricated on self-sensing cantilever, were used in quasi-static conditions (5 × 5 nm2 scan range) with a constant force load of 50 nN.

scanning in contact mode. Postgrowth electron-beam curing with a dose around 150 nC μm−2 was introduced as counterstrategy, which significantly increased the wear resistance of the apex region. Fully optimized 3DNPs revealed high AFM image qualities up to 80 μm s−1 scan speeds and minor wear effects after continuous scanning for several hours (∼50 cm total scan length). In the last step, 3DNPs were fabricated on self-sensing cantilevers, electrically operated in constant current mode, and tested on calibrated heating chips. The results reveal a negative temperature coefficient of resistance of −(7.5 ± 0.2) × 10−3 K−1, which compares very well to alternative resistive probe concepts. This also holds for the noise level and dynamic sensing rates of ±0.5 K and 30 ms K−1, respectively. By that, the concept of freestanding, FEBID-based 3D nanothermistors for temperature sensing, now denoted as 3D thermal nanoprobes, is fully validated. Finally, two more advantageous details should be highlighted in comparison to alternative resistive probe concepts: (1) the achievable apex radii are in the range of 10 nm without any further process steps, which is an essential prerequisite toward high-resolution imaging; and (2) effective process times can be as low as 10 min for both initial direct-write 3D fabrication and electron-beam curing, which eliminates more complicated multistep procedures during cantilever/nanoprobe manufacturing. As mentioned in the introduction, current activities focus on the integration of our 3DNPs in SThM applications, including advanced electronic readouts as well as morphological and thermal resolution tests, to explore the full potential of our nanoprobe approach in combination with GETec’s novel in situ AFM equipment for seamless integration in SEM, FIB, and DBM systems.

4. METHODS 4.1. Finite-Element Simulations. Simulations were conducted using the stationary studies option of the COMSOL Structural Mechanics Module. All models were generated within the COMSOL environment, assuming homogeneous material parameters. Density and Young’s modulus as input parameter were determined in a previous study to be 4 g cm−3 and 14 GPa, respectively.31,33 After a brief investigation on the influence of the Poisson ratio, this parameter was set to 0.2, as it would not change the final stiffness by more than 1% assuming a value lower than 0.5 (see Section 1, Supporting Information). The mesh settings were chosen to “finer”, which indicated a change of less than 1% from “extra fine”, as determined by initial simulations on a pillar test geometry. Structures were deflected in certain directions using an applied force of 10 nN. For better visibility, the deflection of structures was increased in the settings to emphasize and understand the qualitative bending behavior. Stiffness values from simulations were extracted as the ratio of simulated deflection over applied force. 4.2. Nanofabrication. Focused ion beam (FIB) processing and 3D nanoprinting via focused electron-beam-induced deposition (FEBID) were performed in a NOVA 200 Dual Beam Microscope (Thermo Fisher Scientific). All deposits used trimethyl(methylcyclopentadienyl)platinum(IV) (MeCpPt(IV) Me3 ; CAS: 94442-22-5) as precursor, using a standard gas injection system (GIS) mounted in a high-angle port at 38°, arranged 100 μm above the sample surface and 340 μm radial distance to the beam center. The precursor was always heated to 45 °C for at least 3 h prior to first experiments. Prior to any deposition, the beam was corrected for stigmatism and focused by initially deposited single dots. Before 3D fabrication was initiated, the GIS valve was opened for at least 3 min to establish a thermodynamic equilibrium. The base pressure of the microscope before deposition was 5−7 × 10−6 mbar, which increased to 0.9−1.3 × 10−5 mbar after opening the GIS valve. Three-dimensional FEBID was conducted using exposure files created via 3BID26 software for a 16-bit patterning generator simply by building a computer-aided

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b04497. Finite-element simulation-related aspects concerning (i) the Poisson ratio, (ii) comparison to analytical solutions, (iii) stiffness scaling behavior in dependency on heights and cross-sectional areas, and (iv) the full set of stiffness simulation for different architectures, heights, and branch diameters (Section 1); compression-related data concerning (i) AFM tip preparation for FEBID modification, (ii) morphological twisting modes via experiments and simulations, and (iii) nonlinear force distance curves via 22665


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(5) Pollock, H. M.; Hammiche, A. Micro-Thermal Analysis: Techniques and Applications. J. Phys. D: Appl. Phys. 2001, 34, R23− R53. (6) Cahill, D. G.; Braun, P. V.; Chen, G.; Clarke, D. R.; Fan, S.; Goodson, K. E.; Keblinski, P.; King, W. P.; Mahan, G. D.; Majumdar, A.; Maris, H. J.; Phillpot, S. R.; Pop, E.; Shi, L. Nanoscale Thermal Transport. II. 2003−2012. Appl. Phys. Rev. 2014, 1, No. 011305. (7) Tovee, P.; Pumarol, M.; Zeze, D.; Kjoller, K.; Kolosov, O. Nanoscale Spatial Resolution Probes for Scanning Thermal Microscopy of Solid State Materials. J. Appl. Phys. 2012, 112, No. 114317. (8) Yu, Y. J.; Han, M. Y.; Berciaud, S.; Georgescu, A. B.; Heinz, T. F.; Brus, L. E.; Kim, K. S.; Kim, P. High-Resolution Spatial Mapping of the Temperature Distribution of a Joule Self-Heated Graphene Nanoribbon. Appl. Phys. Lett. 2011, 99, No. 183105. (9) Zhang, Y.; Hapenciuc, C. L.; Castillo, E. E.; Borca-Tasciuc, T.; Mehta, R. J.; Karthik, C.; Ramanath, G. A Microprobe Technique for Simultaneously Measuring Thermal Conductivity and Seebeck Coefficient of Thin Films. Appl. Phys. Lett. 2010, 96, No. 062107. (10) Desiatov, B.; Goykhman, I.; Levy, U. Direct Temperature Mapping of Nanoscale Plasmonic Devices. Nano Lett. 2014, 14, 648− 652. (11) Tovee, P. D.; Pumarol, M. E.; Rosamond, M. C.; Jones, R.; Petty, M. C.; Zeze, D. A.; Kolosov, O. V. Nanoscale Resolution Scanning Thermal Microscopy Using Carbon Nanotube Tipped Thermal Probes. Phys. Chem. Chem. Phys. 2014, 16, 1174−1181. (12) Kim, K.; Chung, J.; Hwang, G.; Kwon, O.; Lee, J. S. Quantitative Measurement with Scanning Thermal Microscope by Preventing the Distortion Due to the Heat Transfer through the Air. ACS Nano 2011, 5, 8700−8709. (13) Macpherson, J. V.; Unwin, P. R. Combined Scanning Electrochemical-Atomic Force Microscopy. Anal. Chem. 2000, 72, 276−285. (14) Macpherson, J. V.; Unwin, P. R. Noncontact Electrochemical Imaging with Combined Scanning Electrochemical Atomic Force Microscopy. Anal. Chem. 2001, 73, 550−557. (15) Fischinger, T. J.; Laher, M.; Hild, S. An Evaluation of Local Thermal Analysis of Polymers on the Sub-Micrometer Scale Using Heated Scanning Probe Microscopy Cantilevers. J. Phys. Chem. B 2014, 118, 5570−5576. (16) Despont, M.; Brugger, J.; Drechsler, U.; Dürig, U.; Häberle, W.; Lutwyche, M.; Rothuizen, H.; Stutz, R.; Widmer, R.; Binnig, G.; Rohrer, H.; Vettiger, P. VLSI-NEMS Chip for Parallel AFM Data Storage. Sens. Actuators, A 2000, 80, 100−107. (17) King, W. P.; Bhatia, B.; Felts, J. R.; Kim, H. J.; Kwon, B.; Lee, B.; Somnath, S.; Rosenberger, M. Heated Atomic Force Microscope Cantilevers and Their Applications. Annu. Rev. Heat Transfer 2013, 16, 287−326. (18) Somnath, S.; King, W. P. An Investigation of Heat Transfer between a Microcantilever and a Substrate for Improved Thermal Topography Imaging. Nanotechnology 2014, 25, No. 365501. (19) Dobson, P. S.; Weaver, J. M. R.; Burt, D. P.; Holder, M. N.; Wilson, N. R.; Unwin, P. R.; Macpherson, J. V. Electron Beam Lithographically-Defined Scanning Electrochemical-Atomic Force Microscopy Probes: Fabrication Method and Application to High Resolution Imaging on Heterogeneously Active Surfaces. Phys. Chem. Chem. Phys. 2006, 8, 3909−3914. (20) Dai, Z.; King, W. P.; Park, K. A 100 Nanometer Scale Resistive Heater−Thermometer on a Silicon Cantilever. Nanotechnology 2009, 20, No. 095301. (21) Dai, X.; Moffat, J. G.; Wood, J.; Reading, M. Thermal Scanning Probe Microscopy in the Development of Pharmaceuticals. Adv. Drug Delivery Rev. 2012, 64, 449−460. (22) Jeong, W.; Kim, K.; Kim, Y.; Lee, W.; Reddy, P. Characterization of Nanoscale Temperature Fields during Electromigration of Nanowires. Sci. Rep. 2015, 4, No. 4975. (23) Sadat, S.; Tan, A.; Chua, Y. J.; Reddy, P. Nanoscale Thermometry Using Point Contact Thermocouples. Nano Lett. 2010, 10, 2613−2617.

experiments and simulations (Section 2); AFM imaging concerning (i) high-speed scanning with as-deposited tetrapods, (ii) stiffness and linearity for fully optimized tetrapods, and (iii) long-time/distance scanning with ideally fabricated and post-treated tetrapods (Section 3); and aspects during thermal response studies concerning (i) resistance changes during postgrowth electron-beam curing, (ii) MEMS heater setup, and (iii) cross-influences of force load on electric resistances (Section 4) (PDF) Twisting effects during in situ compression experiments (AVI) Compression behavior of optimized tetrapods (AVI) Real-time SEM imaging during AFM scanning in vacuum using tetrapod-modified AFM tips (AVI)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jürgen Sattelkow: 0000-0002-2767-6145 Johannes E. Fröch: 0000-0003-1111-9704 Robert Winkler: 0000-0001-6088-087X Stefan Hummel: 0000-0002-4426-3609 Christian Schwalb: 0000-0003-2730-8059 Harald Plank: 0000-0003-1112-0908 Present Address ¶

(J.E.F.) School of Mathematical and Physical Sciences, University of Technology Sydney, Ultimo, NSW 2007, Australia.

Author Contributions ∇

J.S. and J.E. F. have equally contributed to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS H.P., J.S., J.E.F., S.H., and R.W. thank Prof. Ferdinand Hofer for scientific discussions and for financial support concerning instrumentation. They also thank DI Franz Hofbauer and DI Anna Weitzer for support with AFM electronics and manuscript preparation, respectively. Special thanks go to Dr. Ernest Fantner for the long-lasting collaboration and for bringing alive the “Christian Doppler Laboratory for Direct-Write Fabrication of 3D Nano-Probes” in joint efforts. The financial support by the Austrian Federal Ministry of Science, Research and Economy and the National Foundation for Research, Technology and Development is gratefully acknowledged. Financial support was also received from FFG Austria in the frame of the “Beyond Europe” initiative (Project AIM, No. 11056459).

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REFERENCES

(1) Brites, C. D. S.; Lima, P. P.; Silva, N. J. O.; Millán, A.; Amaral, V. S.; Palacio, F.; Carlos, L. D. Thermometry at the Nanoscale. Nanoscale 2012, 4, 4799−4829. (2) Liu, W.; Yang, B. Thermography Techniques for Integrated Circuits and Semiconductor Devices. Sens. Rev. 2007, 27, 298−309. (3) Gomès, S.; Assy, A.; Chapuis, P.-O. Scanning Thermal Microscopy: A Review. Phys. Status Solidi 2015, 212, 477−494. (4) Li, J. F.; Liu, W. S.; Zhao, L. D.; Zhou, M. High-Performance Nanostructured Thermoelectric Materials. NPG Asia Mater. 2010, 2, 152−158. 22666


Research Article


by Postgrowth Electron Irradiation. J. Appl. Phys. 2011, 109, No. 063715. (44) Schwalb, C. H.; Grimm, C.; Baranowski, M.; Sachser, R.; Porrati, F.; Reith, H.; Das, P.; Müller, J.; Völklein, F.; Kaya, A.; Huth, M. A Tunable Strain Sensor Using Nanogranular Metals. Sensors 2010, 10, 9847−9856. (45) Sachser, R.; Porrati, F.; Schwalb, C. H.; Huth, M. Universal Conductance Correction in a Tunable Strongly Coupled Nanogranular Metal. Phys. Rev. Lett. 2011, 107, No. 206803. (46) Wu, H. M.; Stern, La.; Chen, J. H.; Huth, M.; Schwalb, C. H.; Winhold, M.; Porrati, F.; Gonzalez, C. M.; Timilsina, R.; Rack, P. D. Synthesis of Nanowires via Helium and Neon Focused Ion Beam Induced Deposition with the Gas Field Ion Microscope. Nanotechnology 2013, 24, No. 175302. (47) Friedli, V.; Utke, I.; Mølhave, K.; Michler, J. Dose and Energy Dependence of Mechanical Properties of Focused Electron-BeamInduced Pillar Deposits from Cu(C5HF6O2)2. Nanotechnology 2009, 20, No. 385304. (48) Huth, M.; Porrati, F.; Schwalb, C.; Winhold, M.; Sachser, R.; Dukic, M.; Adams, J.; Fantner, G. Focused Electron Beam Induced Deposition: A Perspective. Beilstein J. Nanotechnol. 2012, 3, 597−619. (49) Gomes, S.; Chapuis, P.-O.; Nepveu, F.; Trannoy, N.; Volz, S.; Charlot, B.; Tessier, G.; Dilhaire, S.; Cretin, B.; Vairac, P. Temperature Study of Sub-Micrometric ICs by Scanning Thermal Microscopy. IEEE Trans. Compon. Packag. Technol. 2007, 30, 424−431. (50) Wielgoszewski, G.; Sulecki, P.; Gotszalk, T.; Janus, P.; Szmigiel, D.; Grabiec, P.; Zschech, E. Microfabricated Resistive High-Sensitivity Nanoprobe for Scanning Thermal Microscopy. J. Vac. Sci. Technol., B: Nanotechnol. Microelectron.: Mater., Process., Meas., Phenom. 2010, 28, C6N7−C6N11. (51) Sinclair, I. Passive Components for Circuit Design; Elsevier, 2001. (52) Lefèvre, S.; Saulnier, J.-B.; Fuentes, C.; Volz, S. Probe Calibration of the Scanning Thermal Microscope in the AC Mode. Superlattices Microstruct. 2004, 35, 283−288. (53) Puyoo, E.; Grauby, S.; Rampnoux, J.-M.; Rouvière, E.; Dilhaire, S. Scanning Thermal Microscopy of Individual Silicon Nanowires. J. Appl. Phys. 2011, 109, No. 024302. (54) Puyoo, E.; Grauby, S.; Rampnoux, J.-M.; Rouvière, E.; Dilhaire, S. Thermal Exchange Radius Measurement: Application to Nanowire Thermal Imaging. Rev. Sci. Instrum. 2010, 81, No. 073701. (55) Wielgoszewski, G.; Sulecki, P.; Janus, P.; Grabiec, P.; Zschech, E.; Gotszalk, T. A High-Resolution Measurement System for Novel Scanning Thermal Microscopy Resistive Nanoprobes. Meas. Sci. Technol. 2011, 22, No. 094023. (56) Pereira, M. J.; Amaral, J. S.; Silva, N. J. O.; Amaral, V. S. NanoLocalized Thermal Analysis and Mapping of Surface and Sub-Surface Thermal Properties Using Scanning Thermal Microscopy (SThM). Microsc. Microanal. 2016, 22, 1270−1280. (57) Geier, B.; Gspan, C.; Winkler, R.; Schmied, R.; Fowlkes, J. D.; Fitzek, H.; Rauch, S.; Rattenberger, J.; Rack, P. D.; Plank, H. Rapid and Highly Compact Purification for Focused Electron Beam Induced Deposits: A Low Temperature Approach Using Electron Stimulated H2O Reactions. J. Phys. Chem. C 2014, 118, 14009−14016. (58) Nečas, D.; Klapetek, P. Gwyddion: An Open-Source Software for SPM Data Analysis. Open Phys. 2012, 10, 181−188.

(24) Rangelow, I. W.; Gotszalk, T.; Grabiec, P.; Edinger, K.; Abedinov, N. Thermal Nano-Probe. Microelectron. Eng. 2001, 57−58, 737−748. (25) Winkler, R.; Fowlkes, J. D.; Rack, P. D.; Plank, H. 3D NanoPrinting via Focused Electron Beams. J. Appl. Phys. 2019, No. 210901. (26) Fowlkes, J. D.; Winkler, R.; Lewis, B. B.; Fernández-Pacheco, A.; Skoric, L.; Sanz-Hernández, D.; Stanford, M. G.; Mutunga, E.; Rack, P. D.; Plank, H. High-Fidelity 3D-Nanoprinting via Focused Electron Beams: Computer-Aided Design (3BID). ACS Appl. Nano Mater. 2018, 1, 1028−1041. (27) Winkler, R.; Lewis, B. B.; Fowlkes, J. D.; Rack, P. D.; Plank, H. High-Fidelity 3D-Nanoprinting via Focused Electron Beams: Growth Fundamentals. ACS Appl. Nano Mater. 2018, 1, 1014−1027. (28) Winkler, R.; Schmidt, F.-P.; Haselmann, U.; Fowlkes, J. D.; Lewis, B. B.; Kothleitner, G.; Rack, P. D.; Plank, H. Direct-Write 3D Nanoprinting of Plasmonic Structures. ACS Appl. Mater. Interfaces 2017, 9, 8233−8240. (29) Keller, L.; Al Mamoori, M. K. I.; Pieper, J.; Gspan, C.; Stockem, I.; Schröder, C.; Barth, S.; Winkler, R.; Plank, H.; Pohlit, M.; Müller, J.; Huth, M. Direct-Write of Free-Form Building Blocks for Artificial Magnetic 3D Lattices. Sci. Rep. 2018, 8, No. 6160. (30) Al Mamoori, M.; Keller, L.; Pieper, J.; Barth, S.; Winkler, R.; Plank, H.; Müller, J.; Huth, M. Magnetic Characterization of DirectWrite Free-Form Building Blocks for Artificial Magnetic 3D Lattices. Materials 2018, 11, No. 289. (31) Arnold, G.; Winkler, R.; Stermitz, M.; Orthacker, A.; Noh, J.-H.; Fowlkes, J. D.; Kothleitner, G.; Huth, M.; Rack, P. D.; Plank, H. Tunable 3D Nanoresonators for Gas-Sensing Applications. Adv. Funct. Mater. 2018, 28, No. 1707387. (32) Fowlkes, J. D.; Winkler, R.; Lewis, B. B.; Stanford, M. G.; Plank, H.; Rack, P. D. Simulation-Guided 3D Nanomanufacturing via Focused Electron Beam Induced Deposition. ACS Nano 2016, 10, 6163−6172. (33) Lewis, B. B.; Mound, B. A.; Srijanto, B.; Fowlkes, J. D.; Pharr, G. M.; Rack, P. D. Growth and Nanomechanical Characterization of Nanoscale 3D Architectures Grown: Via Focused Electron Beam Induced Deposition. Nanoscale 2017, 9, 16349−16356. (34) Cho, S.; Chasiotis, I.; Friedmann, T. A.; Sullivan, J. P. Young’s Modulus, Poisson’s Ratio and Failure Properties of Tetrahedral Amorphous Diamond-like Carbon for MEMS Devices. J. Micromech. Microeng. 2005, 15, 728−735. (35) Pastorelli, R.; Ferrari, A. C.; Beghi, M. G.; Bottani, C. E.; Robertson, J. Elastic Constants of Ultrathin Diamond-like Carbon Films. Diamond Relat. Mater. 2000, 9, 825−830. (36) Oberg, E.; Jones, F. D.; Horton, H. L.; Ryffel, H. H. Machinery’s Handbook (27th Edition) & Guide to Machinery’s Handbook; Industrial Press, 2004. (37) Kreith, J.; Strunz, T.; Fantner, E. J.; Fantner, G. E.; Cordill, M. J. A Versatile Atomic Force Microscope Integrated with a Scanning Electron Microscope. Rev. Sci. Instrum. 2017, 88, No. 053704. (38) Beer, F. P.; Johnston, E. R., Jr. Mechanics of Materials, 2nd ed.; McGraw-Hill: New York, 1992. (39) Mutunga, E.; Winkler, R.; Sattelkow, J.; Rack, P. D.; Plank, H.; Fowlkes, J. D. Impact of Electron-Beam Heating during 3D Nanoprinting. ACS Nano 2019, 5198−5213. (40) Plank, H.; Haber, T.; Gspan, C.; Kothleitner, G.; Hofer, F. Chemical Tuning of PtC Nanostructures Fabricated via Focused Electron Beam Induced Deposition. Nanotechnology 2013, 24, No. 175305. (41) Winkler, R.; Geier, B.; Plank, H. Spatial Chemistry Evolution during Focused Electron Beam-Induced Deposition: Origins and Workarounds. Appl. Phys. A 2014, 117, 1675−1688. (42) Plank, H.; Kothleitner, G.; Hofer, F.; Michelitsch, S. G.; Gspan, C.; Hohenau, A.; Krenn, J. Optimization of Postgrowth Electron-Beam Curing for Focused Electron-Beam-Induced Pt Deposits. J. Vac. Sci. Technol., B: Nanotechnol. Microelectron.: Mater., Process., Meas., Phenom. 2011, 29, No. 051801. (43) Porrati, F.; Sachser, R.; Schwalb, C. H.; Frangakis, A. S.; Huth, M. Tuning the Electrical Conductivity of Pt-Containing Granular Metals 22667


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