Quantum coherence in a processable vanadyl complex: new tools for the search of molecular spin qubits

A multitechnique investigation of an evaporable vanadyl spin system with long-lived quantum coherence that self-assembles on gold.

Synthesis and samples preparation. The whole synthesis was performed in inert atmosphere following a reported procedure. 1 Vanadium sulphate tetrahydrate (0.5 g, 2.13 mmol) was dissolved in 4 ml of H 2 O milliQ and 8 ml of EtOH with magnetic stirring. Hdpm (1 g, 5.45 mmol) was added and the solution quickly became dark green. Finally, 3 ml of sodium carbonate 0.1 M was mixed with the solution and a solid appeared immediately. The latter was filtered out and washed with water. The powder was purified by sublimation thanks to which we obtained long green crystals.
Anal. Calcd. (found) for [C 22 H 38 O 5 V]: C, 60.96 (61.07); H 8.84 (9.00). Crystals were checked by X-ray diffraction and resulted to correspond to the structure available in the Cambridge Structural Database record CCDC 230339. 1 Diluted samples were also prepared to evaluate the effect of intermolecular interactions on magnetization dynamics. Glassy dispersions were obtained by dissolving VO(dpm) 2 in a 2:3 toluene:CH 2 Cl 2 solution to obtained a glass at low temperature or by dispersing the complex in polystyrene films.
AC susceptometry. A Quantum Design PPMS equipped with AC susceptibility probe (working in the range 10 Hz-10 kHz) and a Quantum Design MPMS SQUID magnetometer (0.1 Hz -1 kHz), have been used to measure the magnetic susceptibility over an extended frequency. The higher sensitivity of the latter set-up allowed to characterize the diluted samples. A variable static field was applied parallel to the oscillating field. Through the Debye model (equation 1) we extrapolated the relaxation time, , and the width of the distribution of the relaxation time, .
where ''() is the imaginary susceptibility,  the angular frequency,  T the isothermal susceptibility and  S the adiabatic susceptibility.  b a EPR Spectroscopy. CW X-Band EPR spectra of all samples were recorded on a Bruker Elexsys E500 spectrometer equipped with a SHQ cavity ( = 9.47 GHz, Florence). Low temperature measurements were obtained using an Oxford Instruments ESR900 continuous flow helium cryostat. Pulsed EPR measurements (1PS 1:10 , 1sol 200mM, 1sol 1mM, 1sol 1mM D ) were performed with a Bruker Elexsys E580 at X-band (Turin,  = 9.75 GHz) equipped with a flexline dielectric ring ENDOR resonator (Bruker EN 4118X-MD4). Temperatures between 4 and 200 K were obtained with an Oxford Instruments CF935 continuous flow helium cryostat. Typical pulse lengths were 16 ns (/2) and 32 ns (). For Echo detected field swept EPR spectra, the Hahn Echo pulse sequence , with fixed interpulse delay time t d =200 ns, was applied while sweeping the magnetic field.
Phase memory times measurements were obtained by measuring the primary echo decay with varying interpulse delay starting from t d =98 ns at a fixed magnetic field. Spin-lattice-relaxation times were measured using the standard inversion recovery sequence (-t w -/2t d -t decho) and by observing the variation of the amplitude of the primary echo as a function of the repetition rate (echo saturation by fast repetition). This second method was find to be more convenient at low temperature (T<40 K) due to the very long T 1 of the sample. Nutation measurements were performed with a nutation pulse (t p ) of variable length followed by a Hahn echo       UHV deposition and characterization. The ML and sub-ML deposition and characterization of VO(dpm) 2 films were performed in situ. The substrate employed was an Au(111) single crystal. The surface was cleaned by repeated Ar + sputtering (2 µA, 1 keV) and annealing (720 K) cycles.

b) Comparison of T 1 values of the central line measured with echo saturation by fast repetition (purple squares) and inversion recovery mode (black triangles).
Considering that VO(dpm) 2 like other β-diketonates shows high volatility, 3 the sublimation was performed in a dedicated preparation chamber with a base pressure of 1×10 −7 mbar; this chamber is directly connected to the XPS and STM chambers. Low coverages were obtained by keeping the molecular powders, hosted in a quartz crucible, at room temperature. The comparison of the STM and XPS characterization of an in situ monolayer deposition performed by heating the powders (at 373 K, where a rate is observed by QCM) or leaving them at room temperature, proves that there is no difference between the two. During the sublimation, the substrate was kept at room temperature.
A K-type thermocouple, buried into the molecular powder, allowed for temperature control.
The thick film was prepared in a home-made evaporation chamber, and transferred to the XPS chamber using a glove bag filled with nitrogen. The sublimation was performed on top of a polycrystalline Au film evaporated on Mica. Preliminarily to the sublimation a hydrogen flameannealing procedure was adopted in order to clean the ex situ prepared substrate. The deposition was performed using the same evaporator as for the monolayer coverage but the powders were heated at 373 K.
The STM images were obtained by an UHV scanning tunneling microscope (Omicron VT-STM) operating at 30 K in the constant current mode with electrochemically-etched W tips. The applied tip bias voltage and the tunneling current of each image are given in the figure caption.
The height estimation of the VO(dpm) 2 molecules was carried out by plotting the height distribution of selected regions (see highlighted areas Figure S12). The measured heights of each region were then fit with two Gaussian functions, one for molecular domains and a second for the background.
The height of the molecular domain is estimated as the difference between the peak positions of the two distributions. We then computed the mean height value averaging over the five considered areas. The error on the mean value, s, is then given by where σ k is the standard deviation of height computed for the k-th area and N is the total number of the considered values.
XPS measurements were carried out in an UHV chamber with a base pressure in the low 10 −10 mbar range. The chamber is equipped with a SPECS Phoibos 150 electron analyzer, a standard Al source and a monochromatic Al X-ray source. The X-ray sources were assembled at 54.44° with respect to the analyzer. For the characterization of the monolayer deposition, we used the monochromatic Al source operating at a power of 100 W (13 kV and 7.7 mA). The characterization of the thick film was performed with a standard Al source with a power of 260 W (13 kV and 20 mA). The pass energy was set to 40 eV for all the experiments.
The monolayer was subjected to differential charging and the Au 4p 3/2 peak present the same shift as the main ones of the molecule (O 1s, V 2p, C 1s). Performing the calibration using the Au 4p 3/2 , the position of the main peak of C 1s was 285.4 eV. In order to use the same calibration for the thin and the thick film, being the Au 4p 3/2 not visible, the methyl C 1s at 285.4 eV signal has been used as a reference to correct the charging effect.
XPS data analysis have been performed by removing the inelastic background by means of the Shirley method 4 and then deconvoluting the experimental spectra using mixed Gaussian and Lorentzian line shapes for each component in the spectra. In the case of V 2p component the adopted method resulted in line with previous literature reports. 5 The background for the O 1s peak was obtained by subtracting also the contribution of the Au 4p 3/2 peak at 547 eV.  Binding Energy (eV) Figure S14. Comparison of O 1s and V 2p XPS spectra of the monolayer and thick film. The thick film of 150 nm was prepared ex situ and transferred into the XPS chamber using a glove bag filled with nitrogen; the monolayer was prepared and kept in situ. The thick film was exposed to air for a variable time reported in the legend.