Effect of nanostructuration on the spin crossover transition in crystalline ultrathin films

Film thickness and microstructure critically affect the spin crossover transition of a 2D coordination polymer.


Substrate preparation and SAM functionalization.
Prior to evaporation of Au, silicon substrates were soaked in a freshly prepared solution of H2O2/NH4OH/H2O (1:1:2) and sonicated for 10 min. This treatment was repeated three times. Next, they were rinsed with Milli-Q water, sonicated 5 min in Milli-Q water twice, and dried under a nitrogen stream. A 3 nm Ti layer followed by the Au layer were then evaporated in an Edwars Auto 500 thermal evaporator in a tungsten basket coated with Al2O3 placed inside a nitrogen glovebox. Base pressure was 2·10 -6 mbar, and evaporation rate 0.02 nm·s -1 . 15 nm of Au was the preferred thickness except for infrared spectroscopy studies where 150 nm were used. For magnetic measurements Mylar was used as diamagnetic substrates, prior to evaporation they were cleaned by sonication in isopropanol for 10 min and then dried under a nitrogen stream. For SAM functionalization substrates were previously activated via O2 plasma treatment (MiniPCFlecto, Plasma Technology), and then immersed in a 1 mM ethanol solution of py-SH for 24 hours. Next, substrates were rinsed with fresh ethanol and dried under a N2 stream.

Infrared reflection absorption spectra (IRRAS).
Measurements were performed on a VeeMax II sampling stage (Pike Technologies) placed in the sample compartment of a Nicolet 5700 Transformation-Infrared Spectrometer. All measurements were performed under nitrogen atmosphere. Each FT-IR spectrum represents the average of 512 scans with a resolution of 4 cm -1 . The infrared beam (incidence angle: 75º) was p-polarized by mean of a manual ZnSe polarizer, and the output signal was collected using a refrigerated mercury cadmium telluride (MTC) detector.

Variable-temperature Raman spectroscopy (VT-Raman).
Measurements were performed on a Horiba T64000 Raman spectrometer. Sample was inserted into a continuous flow micro-cryostat (Cryovac) with a Supplementary Information base pressure of 10 -6 mbar to perform temperature dependent measurements from room temperature to 79 K. Raman spectra were excited at 458 nm using an Ar ion laser and a beam power of 30 mW was maintained over the sample. A grating spectrograph of 1800 gr·mm -1 was used, the entrance slit was kept at 40 µm during measurements to obtain a spectral resolution of about 0.8 cm -1 . Spectra were recorded using an accumulation time of 3600 s due to the low signal intensity.
Atomic force microscopy (AFM). Measurements were performed with a Digital Instruments Veeco Nanoscope IVa microscope in tapping mode using silicon tips with a natural resonance frequency of 300 KHz and with an equivalent constant force of 40 N·m -1 . Scan rate was adjusted during the scanning of each image (0.1-1 Hz, 512 samples/line). Film thickness was evaluated by scratching off the deposited MOF film using a toothpick.

X-ray Photoelectron Spectroscopy (XPS).
Measurements were performed ex situ at the X-ray Spectroscopy Service at the Universidad de Alicante using a K-Alpha X-ray photoelectron spectrometer system (Thermo Scientific). All spectra were collected using Al Ka radiation (1486.6 eV), monochromatized by a twin crystal monochromator, yielding a focused X-ray spot (elliptical in shape with a major axis length of 400 µm) at 3 mA·C and 12 kV. The alpha hemispherical analyser was operated in the constant energy mode with survey scan pass energies of 200 eV to measure the whole energy band and 50 eV in a narrow scan to selectively measure the particular elements. XPS data were analysed with Avantage software. A smart background function was used to approximate the experimental backgrounds. Charge compensation was achieved with the system flood gun that provides low energy electrons and low energy argon ions from a single source. Spectra were referenced using the C 1s main peak (285 eV).

Scanning Electron Microscopy (SEM).
Particle morphologies and dimensions were studied with a Hitachi S-4800 scanning electron microscope at an accelerating voltage of 20 keV, over metalized samples with a mixture of gold and palladium during 30 s.

Powder x-Ray Diffraction (PXRD).
Patterns were collected for polycrystalline samples using 0.5 mm glass capillaries which were mounted, aligned and measured in a PANalytical Empyrean diffractometer (Bragg-Brentano geometry) using copper radiation (Cu Ka = 1.5418 Å) with an PIXcel detector, operating at 40 mA and 45 kV. Profiles were collected by using a Soller Slit of 0.02° and a divergence slit of ¼ at room temperature in the angular range 3° < 2θ < 50° with a step size of 0.013°. LeBail refinements were carried out with the FULLPROF software package. 1 2D Grazing Incidence X-Ray Diffraction (2D-GIXRD). Synchrotron 2D-GIXRD measurements were performed at beamline I07 at Diamond synchrotron (proposal SI14495). Samples were placed into a closed cell mounted on a multiaxis diffractometer and measured under a continuous He flow. Single shot images (1 second exposition) were recorded at room temperature using a Pilatus 2M area detector with wavelength of 0.85 Å (beam energy of 14.53 keV), a sample-to-detector distance of 456 mm and an incidence angle of 0.1°. Data was analysed and processed using DAWN software. 2 Magnetic measurements. Variable-temperature magnetic susceptibility data were recorded on a Quantum Design MPMS-XL-5 SQUID magnetometer equipped with a 7 T magnet, operating at 1 T and a temperature range of 1.8−400 K. Experimental susceptibilities were corrected for diamagnetism of the constituent atoms by the use of Pascal's constants. Mylar was used as a diamagnetic substrate for thin film measurements. In bulk samples the HS fraction at 300 K was adjusted to 100% based on previous Mössbauer spectroscopy studies 3,4 and the Fe II -N distances reported in the crystal structure of [Fe(py)2{Pt(CN)4}] at 298 K. 5 X-ray absorption spectroscopy (XAS). Iron L2,3-edge XAS measurements were performed at the DEIMOS beamline 6 at SOLEIL synchrotron facility and at the BOREAS (BL-29) 7 beamline at ALBA synchrotron using circular polarized light. Samples were irradiated with a beam spot size of 800 x 800 µm 2 in ultrahigh vacuum conditions (10 -10 mbar) at a temperature range of 295-4 K. The beam spot was maintained at approximately the same spot for the whole study by compensating for the temperature change drift in the sample position. A magnetic field up to 1 T was applied perpendicular to the plane. The signal from the sample was detected in total electron yield (TEY) mode. The time required to acquire one spectrum was 3 min. We averaged 8 consecutive XAS spectra to obtain improve signal-to-noise ratio.

Determination of the HS fraction (γHS) in
According to SQUID data ( Figure S10) the HS fraction at 300 K and 100 K for the bulk sample 3 are 100% and 3.5%, respectively. As a result, the γHS can be written as follow: 78 #$%& = 3.5 + 96.5 · Before the fitting, a linear background was subtracted to all spectra and the following normalization constraint was applied:  Peak assigned with (*) was found to be characteristic of the film deterioration due to the long exposition time required to record spectra with acceptable signal-to-noise ratio. A laser wavelength of 458 nm was used to record the 90-cycles film spectrum and a laser wavelength of 638 nm was used to record the bulk spectrum.

Figure S6
. 2D-GIXRD data processing consisted of transforming the real space image (a) into a Q-space image (b). Then 1D XRD profiles were extracted by simple sector integration of the masked Q-space image. Thus, in-plane (from 150º to 180º, c) and out-of-plane (from 60º to 120º, d) diffractograms were obtained. Figure S7. Raman spectra at 300 K (black) and 79 K (red) of a 90-cycles film onto Si/Ti (3 nm)/Au (15 nm). Spectra have been vertically shifted to better appreciate peaks position. Inset shows the normalized ν(CN) vibration around 2200 cm -1 which is highlighted in grey in the full spectra. A displacement of the peaks of 7.5 cm -1 can be observed with temperature. Moreover, a small shift towards higher frequencies is also measured in the ring stretching mode of the pyridine molecule around 1020 cm -1 and an enhancement is observed in the bending (∂) and stretching (ν) modes of the pyridine molecule at 1225 cm -1 and 1610 cm -1 in the LS state. The peak marked with (*) was found to be characteristic of a deterioration of the sample during measurement that could not be avoided due to the long exposition time required to record a spectrum with an acceptable signal-to-noise ratio.   From left to right: SEM images with their respective calculated particle size, PXRD diffractograms with the corresponding crystallite size as calculated using Scherrer's equation ([111] reflection) and the unit cell parameters that resulted from the LeBail profile fitting, and temperature-dependent magnetic susceptibility SQUID measurements with the calculated hysteresis (Hys) and transition temperatures (T1/2). Scherrer's law was not applied to bulk-1, as this equation is only valid for a smaller crystallite size range. [8][9][10] Figure S11. LeBail refinements of bulk [Fe(py)2{Pt(CN)4}] samples.     Figure S18. Device fabrication. a) Optical image of Au (50 nm)/[Fe(py)2{Pt(CN)4}] (90 layers)/Au (70 nm) vertical cross junctions fabricated over a Si/SiO2 (285 nm) wafer with a diameter of 2". First, bottom Au electrodes were deposited by shadow mask on the Si/SiO2 wafer using a Createc MBE evaporator with a base pressure of 10 -9 mbar. Then, wafer was removed from the evaporator and a 90 layer [Fe(py)2{Pt(CN)4}] film was grown by sequential immersion over the Au electrodes previously functionalized as described in the main text. Finally, wafer was transferred again inside the evaporator and top Au electrode was evaporated by shadow mask to fabricate a cross junction configuration. As it can be observed in the picture, more than 80 cross junctions were fabricated at the same time with cross sections of: 50 x 50 µm 2 , 100 x 100 µm 2 and 500 x 500 µm 2 . b) IV curve measured on a cross junction with area of 50 x 50 µm 2 of sample shown in (a). A probe station in a 2-points configuration was used to test all the fabricated junctions. A bias voltage was applied using a Yokogawa GS200 voltage source and current through the junction was measure using a Keithley 6517B electrometer. All the junctions fabricated presented a short-circuited behaviour as the one shown in (b).