Tracking the Verwey Transition in Single Magnetite Nanocrystals by Variable-Temperature Scanning Tunneling Microscopy Amir Hevroni, † Mukund Bapna, ‡ Stephan Piotrowski, ‡ Sara A. Majetich, ‡ and Gil Markovich* ,† † School of Chemistry and Center for Nanoscience and Nanotechnology, Tel Aviv University, Tel Aviv 69978, Israel ‡ Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States * S Supporting Information ABSTRACT: Variable-temperature scanning tunneling spectroscopy revealed a sharp Verwey transition in individual ∼10 nm magnetite nanocrystals prepared by the coprecipitation technique and embedded in the surface of a gold film. The transition was observed as a significant change in the electronic structure around the Fermi level, with an apparent band gap of ∼140−250 meV appearing below the transition temperature and a pseudogap of ∼75 ± 10 meV appearing above it. The transition temperature was invariably observed around 101 ± 2 K for different nanocrystals, as opposed to 123 K typically reported for stoichiometric bulk crystals. This suggests that the lowering of the transition temperature is an intrinsic finite size effect, probably due to the presence of the surface. A dvances in fabrication and scanning probe techniques have enabled detailed studies of electronic phase transitions in nanoscale systems. 1,2 The Verwey transition in Fe 3 O 4 was the first to be associated with charge ordering, 3 followed by studies of electronic transitions in other metal oxides, 4 metal chalcogenides, 5 as well as organic conductors. The Verwey transition was described as a first-order metal−insulator transition accompanied by a structural phase transition where the (T > 123 K phase) cubic symmetry of the Fe 3 O 4 crystal is broken by a small lattice distortion on cooling through the transition temperature. 6 Since Verwey’s seminal paper in 1939, many aspects of the Verwey transition in bulk magnetite crystals have been studied. 7 It is understood that the driving forces for the transition are the strong electron−electron and electron−lattice interactions in the system. Long-range charge ordering is believed to dominate below the transition temperature (T V ), while short-range order is sustained well above it. 8 The long-range order manifests itself by opening a gap in the electronic density of states (DOS) around the Fermi energy level (E F ). This gap has been detected through various methods, 9−11 including tunneling spectroscopy. 12,13 Some photoemission experiments suggest that a reduced gap in the DOS, attributed to short-range ordering, still exists well above T V . 9,10 More recently, the Verwey transition was observed in assemblies of magnetite nanocrystals (NCs) 14 and in thin films, 15 where the effects of grain size, 16,17 surface, 12 and overall particle shape 18 were studied. Still, the exact nature of the transition and its manifestation at finite nanometric scales remain under debate, particularly the lattice and electronic structures of the crystal in the low-temperature phase. Electronic structure probes such as photoelectron spectros- copy or tunneling spectroscopy are surface sensitive, and the surface electronic structure of magnetite is expected to be modified relative to the bulk, 12,19−21 especially in the case of a long-range charge-ordered phase. Density functional calcula- tions of thin magnetite films reveal a Verwey transition even at subnanometer thickness of (001) oriented films covered with gold. 21 However, these calculations also show that the surface layer of magnetite could be insulating, which further underlines the importance of sampling the interior states of the thin film or NC. Tunneling spectroscopy of single magnetite NCs could potentially probe the interior electronic structure of the NC. This can be accomplished through the double-barrier tunnel junction (DBTJ) configuration, which typically forms when a colloidal nanoparticle is placed on a conducting substrate and probed by a metallic tip of a scanning tunneling microscope (see Figure 1a). The electronic structure of semiconductor quantum dots was previously studied using scanning tunneling microscopy and spectroscopy (STM and STS, respectively) techniques. 22,23 By tunneling electrons of different energies through a NC between a tip and conductive substrate in an asymmetric DBTJ configuration (Figure 1a, R gap ≫ R sub ), one may extract information about the particle’s core density of states (DOS). The technique was shown to be effective in probing spin polarization and magnetization dynamics in individual superparamagnetic NCs. 24,25 However, there have Received: March 22, 2016 Accepted: April 18, 2016 Letter pubs.acs.org/JPCL © XXXX American Chemical Society 1661 DOI: 10.1021/acs.jpclett.6b00644 J. Phys. Chem. Lett. 2016, 7, 1661−1666