Samuel Flores-Torres

Mechanical Control of Spin States in Spin-1 Molecules and the Underscreened Kondo Effect

Spin Control Through Molecular Stretching Molecules with high symmetry, such as metal complexes with several equivalent ligands, can, in principle, have this symmetry broken by stresses that lengthen bonds in one direction. Parks et al. (p. 1370; see the Perspective by Jarillo-Herrero) placed cobalt complexes in a break-junction contact and then applied a mechanical force to slowly open the contact. Low-temperature measurement of differential conductance revealed a splitting of the Kondo peak at zero-applied voltage into two features, which occurred by breaking the degeneracy of S = 1 triplet states. This assignment of the spin state was confirmed by the evolution of splitting with magnetic field and by comparison to theory for a case where the conduction electrons only partially screen the spin states. Controlled stretching of individual transition-metal complexes enables direct manipulation of the molecule’s spin states. The ability to make electrical contact to single molecules creates opportunities to examine fundamental processes governing electron flow on the smallest possible length scales. We report experiments in which we controllably stretched individual cobalt complexes having spin S = 1, while simultaneously measuring current flow through the molecule. The molecule’s spin states and magnetic anisotropy were manipulated in the absence of a magnetic field by modification of the molecular symmetry. This control enabled quantitative studies of the underscreened Kondo effect, in which conduction electrons only partially compensate the molecular spin. Our findings demonstrate a mechanism of spin control in single-molecule devices and establish that they can serve as model systems for making precision tests of correlated-electron theories.

Tuning the Kondo Effect with a Mechanically Controllable Break Junction

We study electron transport through C(60) molecules in the Kondo regime using a mechanically controllable break junction. By varying the electrode spacing, we are able to change both the width and the height of the Kondo resonance, indicating modification of the Kondo temperature and the relative strength of coupling to the two electrodes. The linear conductance as a function of T/T(K) agrees with the scaling function expected for the spin-1/2 Kondo problem. We are also able to tune finite-bias Kondo features which appear at the energy of the first C(60) intracage vibrational mode.

Observation of Electroluminescence and Photovoltaic Response in Ionic Junctions

Electronic devices primarily use electronic rather than ionic charge carriers. Using soft-contact lamination, we fabricated ionic junctions between two organic semiconductors with mobile anions and cations, respectively. Mobile ionic charge was successfully deployed to control the direction of electronic current flow in semiconductor devices. As a result, these devices showed electroluminescence under forward bias and a photovoltage upon illumination with visible light. Thus, ionic charge carriers can enhance the performance of existing electronic devices, as well as enable new functionalities.

Organic light-emitting devices with laminated top contacts

We demonstrate the fabrication of organic light-emitting devices based on a ruthenium complex with indium tin oxide anodes and laminated Au cathodes. Light emission was uniform over the whole device area, indicating a high-quality mechanical and electrical contact. The devices showed no rectification, indicating that the laminated contact was ohmic and caused no damage to the ruthenium complex. Comparison with devices using evaporated Au cathodes confirmed the quality of the lamination process.

Photophysical properties of tris(bipyridyl)ruthenium(II) thin films and devices

Absorption and luminescence spectra as well as luminescence lifetimes have been measured for Ru(bpy)2+3 in solution and in thin films of varying thicknesses, and these properties have been correlated with the efficiency of organic light emitting devices (OLEDs) made of the films. The lifetimes decrease for films below about 50 nm in thickness but are relatively constant for thicknesses above about 100 nm. This behavior is consistent with a model in which quenching is caused both by intrinsic properties of the molecules and by Forster energy transfer between chromophores that carries the excitation to surface layers, where the excitation is more efficiently quenched. The external quantum efficiency of the OLEDs is also found to increase with thickness, approaching 1% for thicknesses near 200 nm.

Direct 120 V, 60 Hz operation of an organic light emitting device

We report on lighting panels based on ruthenium(II) tris-bipyridine complexes that can be sourced directly from a standard US outlet. With the aid of the ionic liquid 1-butyl-3-methylimidazolium, the conductivity of the light emitting layer was enhanced to achieve device operation at a 60Hz frequency. Lighting panels were prepared using a cascaded architecture of several electroluminescent devices. This architecture sustains high input voltages, provides fault tolerance, and facilitates the fabrication of large area solid-state lighting panels. Scalability of the drive voltage, radiant flux, and external quantum efficiency is demonstrated for panels with up to N=36 devices. Direct outlet operation is achieved for panels with N=16, 24, and 36 devices.

Contact issues in electroluminescent devices from ruthenium complexes

We report on the temporal evolution of the current, radiance and efficiency of electroluminescent devices based on films of [Ru(bpy)3]2+(PF6−)2 (bpy is 2,2′-bipyridyl) with various electrodes. Under forward bias (with the bottom electrode wired as the anode) the device characteristics were independent of the electrodes used. The situation was different under reverse bias, where differences were observed in the steady-state as well as in the transient characteristics of devices with different electrodes. The origin of this asymmetry is discussed.

In situ identification of a luminescence quencher in an organic light-emitting device

We have used in situ Raman spectroscopy to identify a luminescence quencher formed during organic light-emitting device operation. Raman spectroscopy revealed that oxo-bridged dimerization occurs during the operation of [Ru(bpy)3]2+(PF6−)2 devices, where bpy is 2,2′-bipyridine. Photoluminescence spectroscopy showed that oxo-bridged dimers such as [Ru(bpy)2(H2O)]2O4+(PF6−)4 effectively quench photoluminescence. Comparison of the Raman spectra from devices with the spectra from prepared blended films of [Ru(bpy)3]2+(PF6−)2 and [Ru(bpy)2(H2O)]2O4+(PF6−)4 demonstrated that sufficient dimerization occurs in the device to account for the luminescence quenching observed upon device driving. Dimerization occurred particularly where oxygen and moisture could penetrate the organic film. Dimerization could be a general failure mode of organic electroluminescent devices that incorporate metal complexes. Understanding failure under device-relevant conditions can lead to the development of materials and devices that are intrinsically more resistant to degradation.

Cascaded light-emitting devices based on a ruthenium complex

We use a transition metal complex to demonstrate a cascaded device architecture in which the same metal electrode acts as an anode for one device and a cathode for its neighbor. This architecture does not require patterning of the organic layer and allows monolithic fabrication of panels that show intrinsic fault tolerance to short circuits and are amenable to scaling to large areas.

Temperature dependence of tris(2,2′-bipyridine) ruthenium (II) device characteristics

We have investigated the temperature dependence of the current, radiance, and efficiency from electroluminescent devices based on [Ru(bpy)3]2+(PF6−)2, where bpy is 2,2′-bipyridine. We find that the current increases monotonically with temperature from 200 to 380 K, while the radiance reaches a maximum near room temperature. For temperatures greater than room temperature, an irreversible, current-induced degradation occurs with thermal cycling that diminishes both the radiance and the photoluminescence (PL) quantum yield, but does not affect the current. The temperature dependence of the external quantum efficiency is fully accounted for by the dependence of the PL quantum yield as measured from the emissive area of the device. This implies that the contacts remain ohmic throughout the temperature range investigated. The quenching of the PL with temperature was attributed to thermal activation to a nonradiative d–d transition. The temperature dependence of the current shows a complex behavior in which tran...