PHYSICAL REVIEW B 91, 035437 (2015) Spin density wave and superconducting properties of nanoparticle organic conductor assemblies Laurel E. Winter,1,* Eden Steven,1,* James S. Brooks,1,† Shermane Benjamin,1 Ju-Hyun Park,2 Dominique de Caro,3,4 Christophe Faulmann,3,4 Lydie Valade,3,4 Kane Jacob,3,4 Imane Chtioui,3,4 Belen´ Ballesteros,5,6 and Jordi Fraxedas5,6 1Department of Physics and National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, USA 2National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, USA 3CNRS, Laboratoire de Chimie de Coordination, 205 route de Narbonne, BP 44099, 31077 Toulouse Cedex 4, France 4Universite´ de Toulouse, UPS, INPT, 31077 Toulouse Cedex 4, France 5ICN2 - Institut Catala de Nanociencia i Nanotecnologia, Campus UAB, 08193 Bellaterra, Barcelona, Spain 6CSIC - Consejo Superior de Investigaciones Cientificas, ICN2 Building, 08193 Bellaterra, Barcelona, Spain (Received 3 November 2014; revised manuscript received 6 January 2015; published 27 January 2015) The magnetic susceptibilities of nanoparticle assemblies of two Bechgaard salts (TMTSF)2PF6 and (TMTSF)2ClO4, have been studied vs temperature and magnetic field. In the bulk these materials exhibit a spin density wave formation (TSDW = 12 K) and superconductivity (Tc = 1.2 K), respectively. We show from inductive (susceptibility) measurements that the nanoparticle assemblies exhibit ground-state phase transitions similar to those of randomly oriented polycrystalline samples of the parent materials. Resistivity and diamagnetic shielding measurements yield additional information on the functional nanoparticle structure in terms of stoichiometric and nonstoichiometric composition. DOI: 10.1103/PhysRevB.91.035437 PACS number(s): 75.50.Tt, 75.30.Fv, 74.70.Kn I. INTRODUCTION or electrocrystallization in the presence of stabilizing agents acting as growth inhibitors [9]. Additional refinements of Restricted geometries have been realized in many materials the synthesis conditions have produced a number of DA such as semiconductors and metals, and often the bulk organic superconductors where strong correlations between properties are modified when the size of a structure approaches the spectroscopic and crystallographic properties of the bulk the characteristic extent of the ground-state order parameter. materials and the nanoparticles have been established [10]. For instance, in semiconductor devices [1] or type-I elemental In light of these advances, the main purpose of the present metal superconductors like aluminum [2,3], the evolution work is to identify the low-temperature thermodynamic from an insulator to a metal or superconductor is observed ground states of the nanoparticle species. An example of the with increasing electron density or film thickness, and in nanocrystalline and nanoparticle structure of (TMTSF) ClO nanoparticles of tin [4] or lead [5,6] a modification of the su- 2 4 isshowninFig.1. High-resolution transmission electron perconducting properties is observed for sizes smaller than the microscope (HR-TEM) studies [10] show that single-crystal coherence lengths. The single-molecule magnet material Mn 12 constituent nanoparticles of approximately 3–5 nm in size (see acetate has also been produced as a thin film [7]. Although the Supplemental Material, Fig. 1 [11]) make up nanoparticles that film retains aspects of the more dramatic magnetic hysteresis TEM images show are on average about 10 times larger (e.g., effects seen in the bulk crystalline materials, modifications average cluster size of 34 nm). These nanoparticles form a fine such as an additional magnetic phase also appear. However, powder (assembly) with a random crystallographic orientation progress in the study of thin-film and nanoparticle geometries (see Supplemental Material, Fig. 2 [11]) that is then used for in the important class of organic superconductors, which are the characterization of the physical properties. predominantly charge-transfer organic salts with relatively We focus on the low-temperature and high-magnetic-field large donor and acceptor organic molecules, has remained properties of two new nanoparticle systems in the class of elusive. This is due to the reluctance of these materials to the Bechgaard salts [12]. The first is (TMTSF) PF (also, naturally form epitaxial layers on substrates; rather, the default 2 6 NP-PF ), which undergoes a spin density wave (SDW) and growth morphology, for instance, via electrocrystallization, 6 metal-insulator transition at T = 12 K, and the second is often the nucleation of small crystallites at arbitrary SDW is (TMTSF) ClO (also, NP-ClO ) which, due to the low orientations. Some progress has been made by evaporation 2 4 4 symmetry of the tetrahedral anion ClO , first undergoes and sublimation methods that produce small patches of ordered 4 an anion ordering transition at T = 24 K, followed by charge-transfer complexes on metallic substrates that can then AO a superconducting transition at T = 1.2K [13]. The main be measured by scanning probe methods [8]. Such sample c goal of the present work is to determine if the nanoparticle geometries and measurements present considerable challenges morphologies of these materials exhibit the same ground states and are not conducive to other characterization methods that as the bulk-single-crystalline materials. We will show that the can probe the ground-state thermodynamics of a nanostructure. ground-state properties of the nanoparticles compare favorably Recently dispersed nanoparticles of donor-acceptor (DA) with their bulk counterparts when the highly anisotropic organic conductors have been synthesized by either oxidation nature of the Bechgaard salt properties are taken into account [14]. (Bulk crystals form in needlelike morphologies where the most conducting axis is the a axis oriented along the *Corresponding authors: [email protected], esteven@ magnet.fsu.edu needle direction, the a-b plane constitutes a conducting layer, †Deceased September 27, 2014. and the c axis is the least conducting direction, normal to 1098-0121/2015/91(3)/035437(7) 035437-1 ©2015 American Physical Society LAUREL E. WINTER et al. PHYSICAL REVIEW B 91, 035437 (2015) (a) (b) (c) Average = 34 nm NP-ClO 70 FWHM = 12 nm 4 60 Gaussian Fit 50 40 Count (#) 30 4 20 ClO 10 0 10 20 30 40 50 60 Size (nm) FIG. 1. (Color online) (TMTSF)2ClO4 nanoparticles. (a) Low-resolution TEM image of nanoparticles dispersed on a substrate. (b) HR-TEM image of nanoparticle structure: dashed envelope, approximate extent of the overall nanoparticle; circles, positions of constituent nanoparticles of 3–5 nm in size. (c) Size distribution of nanoparticles in (a). Nanoparticles of (TMTSF)2PF6 exhibit similar morphologies and size distributions. the layers.) However, an unambiguous demonstration of a Since the Bechgaard salts are highly anisotropic in terms possible alteration of the bulk ground-state properties when of their electronic structure and magnetic field effects [12], the nanoparticle size is less than the ground-state coherence a comparison of the electronic properties and susceptibility lengths will require further studies. of a randomly oriented assembly of nanoparticles (and therefore the constituent nanoparticles) with previous bulk- single-crystal measurements requires an average of the a-, b-, II. RESULTS AND DISCUSSION and c-axis properties. For this purpose, we assume that the For clarity in terminology in what follows, we note that electrical current path or applied magnetic field direction lies the objects on the order of 30–60 nm, as seen in the with equal probability along any of the three principle axes of lower-resolution TEM image [Fig. 1(a)], are referred to as each nanoparticle. This leads to a simple 1/3 contribution of the nanoparticles. These nanoparticles are made up of smaller property from each direction, averaged to allow a comparison entities, referred to as the constituent nanoparticles, with size with the corresponding nanoparticle assembly data. 3–5 nm, as seen from the HR-TEM image [Fig. 1(b)]. For susceptibility and inductive studies, the samples measured A. Spin-density wave transition in (TMTSF)2PF6 nanoparticles were encapsulated powders or assemblies (see Supplemental Material, Fig. 2 [11]) comprised of nanoparticles. For electrical We first discuss the electronic transport of the SDW transport measurements, nanoparticle assemblies were studied transition in the (TMTSF)2PF6 nanoparticle assembly. In in a four-terminal configuration. In the following sections powders, the nanoparticles tend to form a dense, intercon- measurements on the nanoparticle assemblies are presented nected assembly unlike that observed in crushed single-crystal in parallel with comparisons with bulk-single-crystal results. powders where the crystals are well dispersed and separated (Supplemental Material, Fig. 2 [11]). This indicates a strong interparticle interaction between nanoparticles (Supplemental Material, video 1 [11]). The source of this unusual mutual 4 attraction is not presently understood, but based on the sample 3 R SC ) 3 preparation, we do not believe there are residual solvents or R E = 2 meV 2 NP a stabilizing agents on the surface of the nanoparticles. We find Norm RModel 2 the assembly is electrically conducting, and the temperature- dependent resistance of the nanoparticle assembly was studied 10 Ln(R 1 using an adhesive stamp electrode method [15] and a lock-in E = 0.75 meV 6 0 a amplifier (SRS 830) in a four-probe configuration down to Norm 5 R cryogenic temperatures. As observed in Fig. 2, the SDW 4 0.08 0.12 0.16 0.20 transition (accompanied by a metal-insulator transition) occurs 3 1/Temp (1/K) at TSDW = 12 K, similar to that of the bulk single crystals. We 2 RSC note that in bulk single crystals, the temperature-dependent R = R Model S T dR/dT > RP resistance above SDW is metallic, i.e., 0, but the 1 nanoparticle assembly shows very little dependence of the 6 8 10 12 14 16 18 20 resistance above TSDW. However, below TSDW the activated component of the resistance rises quickly and is clearly Temperature (K) observable above the background. Notably, the activation energy (E )oftheNP-PF assembly FIG.