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Atomic Imaging of Phase Transitions and Morphology Transformations in Nanocrystals By Marijn A. van Huis,* Neil P. Young, Gre´gory Pandraud, J. Fredrik Creemer, Danie¨l Vanmaekelbergh, Angus I. Kirkland, and Henny W. Zandbergen

The behavior of crystalline material changes dramatically once its dimensions are reduced towards the nanometer scale.[1] Inevitably, a large fraction of all atoms are positioned at the surface or interface and as a consequence many structural transformations are driven by the tendency to reduce the surface or interfacial energy of the system.[2] In comparison to bulk matter, kinetic barriers can differ significantly, as the driving forces are limited by the much shorter length scales. The study of structural transitions in nanometer-sized systems also has industrial impact. For instance, many catalytic systems[3] are based on specific atomic configurations, which is also true for miniaturized opto-electrical components based on semiconductor quantum dots, rods, or wires.[4] These systems require structural stability in a large temperature region around the working temperature. Although macroscopic systems are usually stable under such conditions, stability becomes a critical issue once the dimensions of the system are reduced to the nanometer scale. High-resolution transmission electron microscopy (HR-TEM) is an indispensable tool in the study of structural changes of nanophase materials. In general, only the final result of a heat or pressure treatment is examined (ex situ TEM).[1a,b,5] However, in order to obtain real-time information on the mechanism and kinetics of temperature-induced structural changes and phase transitions, the heating needs to be performed concurrently with high-resolution imaging (in situ TEM).[6] As aberration-corrected TEM, with its excellent resolution, becomes increasingly prevalent,[7] it has become a key challenge to maintain the superior stability and resolution during in situ heating experiments. We, and other research groups, experienced that conventional heating holders suffer from a thermal drift, which renders atomic-scale resolution impossible for temperatures in higher than 500 K.[8] In order to achieve atomic resolution

at high temperatures, aberration-corrected TEM needs to be combined with the recent technological advances in the fabrication of microhotplates through microelectronic mechanical system (MEMS) technology.[9] Here, we show that by using a MEMS microhotplate specimen holder at a temperature of 1000 K, a superior resolution of 100 pm and a thermal drift of 0.02 nm s–1 after 20 min is achieved in an aberration-corrected Titan microscope operating at 300 kV. The potential of this achievement for the investigation of temperature-induced phase transitions and structural transformations on the atomic scale is demonstrated with two case studies on PbSe and Au nanocrystals, respectively. The development of low-drift heating holders and the performance of the current setup is detailed in the Supporting Information. Figure 1 shows the heating holder containing the MEMS microhotplate and the central heating area, onto which the system under investigation can be deposited. The 1-mm-thick SiN microhotplate contains an embedded, coiled Pt wire that is 200 nm thick and 32 mm wide (Fig. 1d). Electron-transparent, 10-nm-thick SiN membranes are present between the windings

[*] Dr. M. A. van Huis, Dr. G. Pandraud, Prof. H. W. Zandbergen Kavli Institute of Nanoscience, Delft University of Technology Lorentzweg 1, 2628 CJ Delft (The Netherlands) E-mail: [email protected] Dr. N. P. Young, Prof. A. I. Kirkland Department of Materials, University of Oxford Park Road, Oxford OX1 3PH (UK) Dr. J. F. Creemer DIMES-ECTM, Delft University of Technology Feldmannweg 17, 2628 CT Delft (The Netherlands) Prof. D. Vanmaekelbergh Debye Institute for NanoMaterials Science, Utrecht University Princetonplein 1, 3508 TH Utrecht (The Netherlands)

DOI: 10.1002/adma.200902561

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Figure 1. a, b) Optical images of the TEM specimen holder with a MEMS microhotplate that is fabricated using silicon-based fabrication technology [9c]. c,d) The center of the microhotplate contains an embedded, planar Pt wire for local heating. Between the windings are electron-transparent viewing windows (dimensions 5 mm  20 mm), where the SiN is 10–15 nm thick. The scale bar in (d) indicates 100 mm. e) TEM image of two twinned Au NPs recorded at a temperature of 1000 K. The scale bar indicates 5 nm.

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the 6-nm-long plane is stripped within two seconds, while at other times an entire plane is removed within a time frame of 200 ms. Movie S2 shows a sublimating PbSe nanowire at a temperature of 750 K which decreases in size, fragments, and, eventually, completely sublimes. The sublimation happens in the same manner everywhere on the support, this includes also the areas that are not examined with the electron beam and, therefore, irradiation effects do not play an important role. Comparing Movies S1 and S2, it is obvious that the sublimation process at 750 K is considerably faster than at 700 K. Colloidal PbSe nanocrystals capped with oleates exhibit strongly polar {111} facets.[11a,12] The polarity and stability of these planes are much debated.[4a,11] In order to examine the relative stability of the different facets of PbSe nanocrystals with respect to sublimation, PbSe NCs were examined in a [110] projection so that the {001}, {011} and {111} surfaces could be simultaneously observed edge-on. Figure 2b and Movie S3 show a PbSe NC that was heated almost instantaneously (within 20 s) to a temperature of 720 K so that the polar surfaces could not reconstruct after evaporation of the capping ligands. The nonpolar {011} facets are rather small, while the polar {111} and nonpolar {001} surfaces are clearly visible (indicating that initially these two types have similar stability). From the movie, it is clear that the polar {111} surfaces sublimate at a considerably higher rate than the {001} surfaces. Note that unstable facets can grow in area during a certain time of the sublimation process increasing the total energy of the crystal. This is opposite to the growth process of crystals, where fast growing unstable facets disappear while the area of the more stable ones increases. The non-negligible contribution of the surface energy to the total energy of a nanoscale system can result in the co-existence of different phases and morphologies for a single particle. In the case of Au nanoparticles (NPs), minimization of the surface energy can promote strained crystallographic atomic packing, whereby the NP is not a single crystal but consists of twinned subcrystals. Figure 1e shows the TEM image of two such NPs, recorded at a temperature of 1000 K. The particle at the top has a single twin plane at the boundary between two subcrystals, while the bottom one has many subcrystals. The latter type is referred to as multiply twinned particle (MTP); these nano-polycrystals can exhibit symmetrical motifs such as decahedral and icosahedral morphologies.[1a,c,13] Figure 3a shows stills of a 5.5-nm-sized Au NP, heated to just below its melting point at 1000 K. The exposure time is 150 ms and the NP was found to switch rapidly between the different polymorphs – single crystal, single twin, and multiply twinned – with several twinning/ Figure 2. Atomic layer-by-layer sublimation of PbSe NCs. a) Stills of a movie recording displaying detwinning events within a second. The stills the sublimation of a PbSe NC, recorded at a temperature of 700 K. The cubic rocksalt lattice of displayed in Figure 3a can also be observed in PbSe is projected along [100]. The atomic surface planes of the NCs are removed layer by layer. Movie S4, which covers 12 seconds. Movie S5 b) TEM images of a PbSe NC in a near-[110] projection showing nonpolar {001} and {011} shows another example of repeated morpholsurfaces and polar {111} surfaces. The latter sublimate at a faster rate (white arrows). The experimental images are preceded by a schematic showing a PbSe nanocluster in the same ogy changes in a 9-nm Au NP, recorded at a orientation with surfactants stabilizing the polar {111} surfaces. The kinetics of the sublimation temperature of 1040 K. Although structural instability at elevated temperature has been can be observed even more clearly in Movies S1–S3. The scale bars indicate 4 nm.

of the Pt wire for TEM imaging of nanoclusters (NCs). The very low conductive loss of the heater (20 mW at 1000 K) results in a minimal thermal drift, enabling HR-TEM at these temperatures (Fig. 1e). The breaking of the 100-pm barrier for in situ heating work is crucial for atomic-scale studies of materials under transition. Below, we show several in situ nanotransformation experiments, whereby key results are obtained from the high level of structural detail that could be resolved. Semiconductor PbSe NCs and their self-assembled arrays display intriguing optical and optoelectronic properties resulting from quantum confinement.[4b–d,10] PbSe nanocrystals can be obtained in a wide variety of morphologies such as nearly spherical but facetted dots, wires, stars, and rings. The stability of the different crystal planes and their surface charge related to the overall shape of the crystals has been extensively debated, recently.[4a,11] The performance of the microhotplate allowed us to monitor the sublimation of PbSe nanocrystals in situ on an atomic scale. Figure 2a and Movie S1 of the Supporting Information illustrate the sublimation of a PbSe nanocrystal recorded at a temperature of 700 K. A rectangular nanocrystal, 6 nm wide and 10 nm long, is projected along the cubic [100] axis of the PbSe rocksalt lattice. At the top and bottom face of the nanocrystal, atomic PbSe planes are stripped one by one. Hence, sublimation must occur in a molecular fashion, whereby the termination at the newly formed surface is retained. Once a terrace step has formed on a {001} surface (a cubical plane), PbSe units at the step are easily removed. Consequently, sublimation starts at the corner of the NC and then proceeds in one direction until the entire plane is removed, thereby restoring the energetically favorable, defect-free {001} surface. We observed that the rate of sublimation varies in an erratic way: sometimes

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Figure 3. Thermal instability of a 5.5-nm Au NP. Stills of a movie recording displaying thermally induced structural instability of a Au NP. a) Transient structures at a constant temperature of 990 K, just below the melting point at 1000 K. Configurations are indicated as single crystal, twinned, transient structure, and multiply twinned particle (MTP) (see also corresponding Movie S4). b) Reversible melt and recrystallization of the same Au particle within a 10 K temperature window. The scale bars indicate 2 nm.

reported previously,[1a] Figure 3a and Movie S4 and S5 show striking real-time visualizations of rapid structural switching of a single nanocrystal. It is known that intense electron-beam irradiation can also induce twinning;[1a] however, in the present case the very frequent twinning on the onset of melting was also observed at very low electron fluencies (observed at low magnification). Most likely the morphology transformations occur through a cooperative slip-dislocation mechanism as proposed by Koga et al.[13c] The ‘‘morphology switching’’ is an outstanding example of the different thermodynamics of NCs: at elevated temperature, the particles diffuse upon the multidimensional free-energy surface, hopping between various local minima, whereby the morphology can change suddenly with every successful jump. In a subsequent experiment, displayed in Figure 3b, the temperature was increased and decreased repeatedly between 990 and 1000 K. As a result the particle could be molten and recrystallized within a temperature window of 10 K, demonstrating the excellent performance of the heating holder in combination with the temperature-control electronics. The melting temperature of 1000 K for a 5.5-nm NP is slightly lower than the temperature of 1130 K, reported previously by Buffat and Borel.[13d] Such discrepancies can arise because the condition of thermal equilibrium is not easily satisfied for small particles in a vacuum environment, or, for example, because of carbon contamination, which is known to prevent transitions. Furthermore, ‘‘melting’’ as observed by HR-TEM does not necessarily imply that the particle is fully molten. Unlike for macroscopic crystals with a well-defined melting temperature at a given pressure, nanoscale particles have a certain temperature range in which solid and liquid co-exist.[13c,d,14]

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In nanostructured crystalline materials, cooperative structural transformations and phase transitions can occur on a remarkably short time scale and under conditions at which macroscopic crystals are stable. HR-TEM at elevated temperature can be used to probe such transitions at the atomic scale and in real-time, eventually, as a route towards elucidation of the properties of matter at the nanoscale. The atomic resolution, which is enabled by the MEMS technology, is indispensable to observe the kinetic features as demonstrated in this work; future in situ experiments will prove of great importance in developing relations between the structure and the physical and chemical properties of nanomaterials, such as catalysts.

Experimental Microhotplates: The MEMS microhotplates were cleaned by oxygen plasma etching for 20 s before dropcasting the NCs. The temperature was regulated through monitoring the resistance of the Pt wire (4-point measurement, contacts visible in Fig. 1a). Further details regarding the manufacturing process of these microhotplates can be found elsewhere [9c]. The heaters were calibrated by heating the entire microhotplate inside a vacuum tube (pressure 10–3 bar) within a large conventional furnace (ThermConcept KM04/12H, temperature controller Ht40A), while monitoring the resistance of the Pt wire R as a function of temperature T. The R–T curve then served as a reference for the home-built electronics and the software controlling of the voltage across the wire. TEM: The resolution and drift experiments were performed in a cubed Titan microscope with aberration correction, operating at 300 kV. The resolution was determined from FFT (fast Fourier transform) analysis of Au-NC recordings, as detailed in the Supporting Information. The thermal drift was measured by increasing the temperature from room temperature

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Acknowledgements The authors acknowledge financial support from the Stichting Technologie en Wetenschap (STW, project number 07532), the Stichting Fundamenteel Onderzoek der Materie (FOM), and the European Union Framework 6 program, Reference 026019 ESTEEM. We thank G. Hoveling for the construction of the heating holder and D. de Gans for the development of the heating control electronics. H. Xu (Univ. of Wisconsin-Madison, USA) is acknowledged for the synthesis of the Au NPs. Supporting Information is available online from Wiley InterScience or from the author. Received: July 30, 2009 Published online: October 20, 2009 [1] a) L. D. Marks, Rep. Prog. Phys. 1994, 57, 603. b) A. P. Alivisatos, Science 1996, 271, 933. c) T. P. Martin, Phys. Rep. 1996, 273, 199. d) C.-C. Chen, A. B. Herhold, C. S. Johnson, A. P. Alivisatos, Science 1997, 276, 398. [2] a) F. Baletto, R. Ferrando, Rev. Mod. Phys. 2005, 77, 371. b) Y. Yin, A. P. Alivisatos, Nature 2005, 437, 664. [3] a) C. T. Campbell, Science 2004, 306, 234. b) T. W. Hansen, J. B. Wagner, P. L. Hansen, S. Dahl, H. Topsøe, C. J. H. Jacobsen, Science 2001, 294, 1508.

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to 1000 K, waiting for 20 min. and subsequently taking exposures every 2 min for a duration of 30 min. The thermal drift of the holder was then determined by autocorrelation of the images. Movies were recorded on a CM300-UT microscope operating at 300 kV and equipped with a fast-scan Tietz camera set at 512  512 pixel size and on the Titan microscope operating at 300 kV. NPs: Au NPs were prepared through reduction of tetrachloroauric acid (HAuCl3) with sodium citrate and tannic acid [15]. The NPs were kept in a dilute water suspension and were dropcast onto the microheater immediately before the experiment. The dilution was tuned to give well-isolated single particles once dropcast onto the heating chip. The synthesis and properties of the (initially 10-nm-sized) PbSe NCs are described elsewhere [16].

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Atomic imaging of phase transitions and morphology transformations in nanocrystals.

A newly developed SiN microhotplate allows specimens to be studied at temperatures up to 1000 K at a resolution of 100 picometer. Aberration-corrected...
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