Unprecedented Size Effect on the Phase Stability of Molecular Thin

Article Cite This: J. Phys. Chem. C 2017, 121, 25617-25621

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Unprecedented Size Effect on the Phase Stability of Molecular Thin Films Displaying a Spin Transition Victoria Shalabaeva,† Mirko Mikolasek,‡ Maria D. Manrique-Juarez,† Alin-Ciprian Bas,† Sylvain Rat,† Lionel Salmon,† William Nicolazzi,† Gábor Molnár,*,† and Azzedine Bousseksou*,† †

LCC, CNRS and Université de Toulouse, UPS, INP, F-31077 Toulouse, France ESRF-The European Synchrotron, CS40220, 38043 Cedex 9, Grenoble, France

‡

S Supporting Information *

ABSTRACT: An unexpected upshift of the spin transition temperature by ca. 3 K is observed in thermally evaporated films of the [FeII(HB(tz)3)2] (tz = 1,2,4-triazol1-yl) complex when reducing the film thickness from ca. 200 to 45 nm. Fitting the experimental data to continuum mechanics and thermodynamical models allows us to propose an explanation based on the anisotropy of the transformation strain leading to ∼5 mJ/m2 higher 00l surface energy in the high-spin phase.

1. INTRODUCTION The phase stability of bulk materials is inexorably altered at reduced sizes due, primarily, to the increasing role of surfaces/ interfaces. The most well-known example for this phenomenon is the melting point depression of metallic nanoparticles.1 Finite size effects on solid−solid polymorphism has been less investigated, but similar to the melting phenomenon, a decrease (increase) of the transition temperature (pressure) is observed universally.2−10 This change of the P,T-phase diagram is driven fundamentally by the fact that the cohesive energy of high temperature (low pressure) phases is generally lower, which implies also a lower surface energy. Higher-energy polymorphs, which are unstable in the bulk material, thus become thermodynamically stabilized in small particles due to the increasing contribution of the surface energy to the total free energy of the particle. A notable exception to this trend is the melting point elevation of small particles when embedded in a solid matrix with an epitaxial confinement.11 This phenomenon is believed to result from the specific properties of the epitaxial interface, but the mechanistic details often remain elusive. As a new example for such atypical behavior, here we report on the elevation of the spin transition temperature in nanocrystalline thin films of [FeII(HB(tz)3)2] (1) when reducing their thickness. We show that this unusual stabilization of the lowtemperature phase can be linked to the particular anisotropy of the structural changes associated with the low spin (LS) to high spin (HS) transition in 1. In the past decade, the investigation of nanometer-sized molecular spin crossover (SCO) materials has generated significant interest due to their potential applications in © 2017 American Chemical Society

nanoelectronic, nanophotonic, and nanomechanical devices.12−15 Understanding the complex relationships among the size, crystal structure, surface/interface properties, and the SCO behavior of these various nano-objects is arduous but will be crucial for integrating them into useful devices. The general experimental observation,16−19 supported also by various theoretical predictions,20−24 is a loss of the hysteresis properties, the occurrence of incomplete transitions, and a downshift of the transition temperature (i.e., the stabilization of the HS phase) when the size of the objects is reduced. All these observations can be satisfactorily explained on thermodynamical grounds, assuming that the surface energy of these systems is lower in the HS state.25 Unfortunately, at present the link between theory and experiment remains rather poor for different reasons. First of all, the surface energies of SCO compounds are not known, as these quantities are difficult to determine in both spin states. In addition, to produce SCO nano-objects with different sizes it isobviouslynecessary to use different synthesis conditions, which consequently can lead to differences not only in the size of the object, but also in other parameters, such as the particle shape, composition, crystallinity, surface coating, defects, and so forth. Unfortunately, these parameters remain often ill-determined due to the lack of appropriate sample characterization tools. Considering this, it becomes necessary to develop series of high-quality, sizecontrolled samples, which are comparable in terms of Received: October 12, 2017 Revised: October 27, 2017 Published: October 27, 2017 25617

DOI: 10.1021/acs.jpcc.7b10124 J. Phys. Chem. C 2017, 121, 25617−25621

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Figure 1. Experimental data on [Fe(HB(tz)3)2] thin films. (a) Representative AFM topography image of a 45 nm thick film (image size 10 × 10 μm2). (b) Representative UV absorbance spectra of the 200 nm film at selected temperatures. (c) Temperature dependence of the normalized absorbance at 317 nm along the fourth heating−cooling cycle recorded at 1 K/min scan rate for eight films with different thicknesses between 45 and 200 nm. The inset shows the derivatives of the absorbance curves in the heating mode. (d) Absorbance change at 317 nm between the HS and LS states as a function of the film thickness. The line is a linear fit providing the absorption coefficient α. (e, f) Transition temperature and hysteresis width as a function of the film thickness for the different heating−cooling cycles. The dashed lines are guides to the eye.

0.03 Å/s. The evaporation rate and film thickness were monitored in situ by a quartz crystal microbalance. The films were deposited onto fused silica, which were cleaned with acetone and 2-propanol to remove contaminants. The final control of the film thickness and topography was carried out using a SmartSPM AFM microscope (AIST-NT) in amplitudemodulation mode in ambient air using OMCLAC160TS-R3 (Olympus) probes. Temperature-dependent absorbance spectra of the films were collected at wavelengths between 200 and 800 nm using a Cary 50 (Agilent Technologies) spectrophotometer and a Linkam FTIR-600 heating/cooling stage (equipped with fused silica windows). The sample chamber was purged by dry nitrogen, and spectra were acquired in the 293−393 K range with 1 K/min rate.

morphology, chemical composition, crystallinity, and surface/ interface properties. In this context, the recently developed26 thin films of 1 appear as “ideal” samples for the study of size reduction effects. They are prepared by thermal evaporation of the presublimated powder under high vacuum conditions, providing films with very high purity and tight thickness control. In addition, postdeposition recrystallization of 1 allows for highly oriented nanocrystalline films with smooth surfaces and robust, wellreproducible SCO properties. Indeed, from crystallographic, Raman spectroscopic, AFM, and optical absorption data, these films appear identical in all aspects, at least in a thickness range between ca. 40 and 200 nm.26 Their similarity is most clearly attested by the fact that, at a first examination, we have noticed no significant difference between the spin transition curves of films with different thicknesses.26 However, a careful temperature-dependent UV absorption spectroscopic examination revealed an unprecedented thickness dependence, which we report here, together with a theoretical model, which allows us to make a link between sample size, crystal structure, and SCO properties. As such, we believe that this study brings to light key fundamental aspects of the spin transition at the nanoscale.

3. RESULTS AND DISCUSSION Eight thin films of 1 with thicknesses between 45 and 200 nm were prepared by high-vacuum deposition on fused silica substrates. A subsequent treatment for 10 min in air with a relative humidity of approximately 80% was further employed for the formation of stable and fully crystalline layers.26 The film thickness was determined from AFM data [see Figure S1 in the Supporting Information (SI)]. The films are characterized by a homogeneous morphology with an arithmetic average surface roughness (Ra) of ca. 1−2 nm [see Figures 1a and S2 (SI) for AFM surface topography images]. They display also excellent stability upon both thermal cycling and storage in ambient air.26 Variable temperature optical absorption measurements were performed over four heating−cooling cycles between 293 and 393 K with a rate of 1 K/min, with a tight

2. EXPERIMENTAL SECTION Reagents and solvents used in this study are commercially available. The bulk powder [Fe(HB(tz)3)2] was synthesized as described in ref 27. Thin films were grown by thermal evaporation in a PREVAC thermal deposition system at a base pressure of ca. 2 × 10−7 mbar. The bulk powder was heated until 250 °C in a quartz crucible and evaporated at a rate of 25618

DOI: 10.1021/acs.jpcc.7b10124 J. Phys. Chem. C 2017, 121, 25617−25621

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The Journal of Physical Chemistry C control on all experimental conditions (synthesis, pretreatment, storage, and measurement), including even details such as the time lapse between the film deposition and the measurement cycles. Figure 1b shows representative optical spectra, whereas the temperature dependence of the absorbance of films with different thicknesses is plotted in Figure 1c. The abrupt change of the absorbance around ca. 337−338 K with a small hysteresis of ca. 0.5 K width clearly reflects the SCO phenomenon. It is worthwhile to note that the spin transition temperatures of 1 were shown to be strictly rate-independent between 0.1 and 5 K/min;28 i.e., they denote the quasistatic hysteresis. The SCO curves are very similar for different thicknesses: one can observe the same shape and the same small hysteresis, which is a very sensitive indicator of the sample quality. The perfect linear fit of the UV absorbance vs thickness curve (Figure 1d) confirms also the full crystallinity and complete spin transition in each film. Nevertheless, a closer look at the SCO curves reveals a slight, but systematic, increase of the transition temperature for decreasing film thickness, which is particularly obvious in the derivative curves (see the inset of Figure 1c). Parts e and f of Figure 1 show the thickness dependence of the spin transition temperature T1/2 (which we approximate here as the barycenter of the hysteresis loop) and the hysteresis width, respectively. We must note that during the first heating of the sample the spin transition occurs at a slightly higher temperature than in the successive cycles (see Figure S3, SI), which can be possibly attributed to the relaxation of some residual film growth stress. Then, the transition temperatures of the successive cycles become well-reproducible. Hence in Figure 1 the transition temperatures and hysteresis widths corresponding to thermal cycles 2−4 are shown only, as these quantities cannot be defined for the first cycle. The key experimental finding here is the systematic upshift by ca. 3 K of T1/2 when decreasing the film thickness from 200 to 45 nm. What also deserves attention is the fact that the hysteresis width, unlike T1/2, remains virtually the same for different film thicknesses (ca. 0.5 K). As mentioned in the Introduction, the loss of hysteresis and the increase of residual fractions generally accompany the size reduction in SCO materials. However, in the present case, the surface-to-volume ratio does not increase sufficiently to give rise to these phenomena. Before discussing the intrinsic film properties, one should notice that the observed decrease of T1/2 with increasing film thickness might also occur due to the heating of the film by the UV light, which is used to probe its spin state. In order to examine this possibility, we performed UV absorption measurements by placing a ca. 2.6% UV transmission filter in front of the samples to expose them to reduced light intensity. The measurements done with and without the filter gave strictly the same result, demonstrating the absence of any significant thermal effect (see Figure S4, SI). There can be multiple origins of the upshift of the transition temperature. From a mechanical point of view, it may result from a biaxial stress in the SCO film due to its mechanical coupling with the substrate leading to a spin-state-dependent elastic work. From a nanothermodynamical point of view, it may result from excess quantities due to the increasing surfaceto-volume ratio at this scale. In this latter case, the relevant parameters are surface energy and stress, which can provide a driving force for phase transitions, which minimize the surface/ interface energy.6−8 If we take into account these ingredients as well as the bulk free energy, the change in the Gibbs free energy of the film for the LS to HS transition can be expressed as

ΔG(T ) = G HS − G LS = ΔH − T ΔS + ΔγgbAgb + ΔγsvA sv + ΔγssA ss + ΔWel

(1)

where H is the bulk enthalpy, S is the bulk entropy, γ is the surface energy density, A is the surface/interface area, and Wel is the mechanical work due to the elastic interfacial stress at the film/substrate interface. The subscripts gb, sv, and ss stand for the different interfaces (grain boundary, solid−vapor, and solid−solid, respectively). At equilibrium, ΔG(T1/2) = 0, we obtain B T1/2 = T1/2 +

ΔγgbAgb + ΔγsvA sv + ΔγssA ss + ΔWel ΔS

(2)

ΔH

B where T1/2 = ΔS is the bulk transition temperature of the film. Let us examine one by one the role of these parameters. The biaxial stress and the resulting elastic work Wel was first pointed out to be at the origin of the transition temperature shift. Indeed, most thin films are under biaxial mechanical stress, the sign (compressive or tensile) and magnitude of which can be thickness-dependent. In our case we determined the intrinsic film stress using a recently developed approach based on micromechanical devices,29 and we assessed a ca. 320 MPa value. Such high tensile stresses occur frequently in annealed films as the crystallization process results usually in denser films.30 Perhaps even more importantly, the strain associated with the SCO leads also to the variation of the stress state of the film. Indeed, as discussed in refs 26 and 27, the LS to HS transition in 1 leads to an important expansion of the orthorhombic (Pbca) unit cell volume by +4.5%, but this strain is strongly anisotropic: −2.3% for the a-axis, +1.0% for the b-axis, and +5.6% for the c-axis. We have evaluated the work density change associated with these phenomena (i.e., intrinsic and SCO induced stress) using a straightforward continuum mechanics approach, and we found that only a very small shift of the transition temperature, on the order of a few millikelvin, can be attributed to the film stress, which we can thus safely neglect in our case (see the SI for the calculation details). From our previous X-ray diffraction study, the average crystalline domain size was estimated as ca. 45−50 nm for each film, and it was shown that the crystalline domains were oriented with their orthorhombic c-axis normal to the surface.26 Taking into account the similar crystallite sizes, the grain boundary energy (γGB) should not play an important role here;31 all the more, the grain boundaries are likely to consist of low-energy (semi)coherent interfaces. The high-energy solid− vapor and film−substrate interface energies should therefore account primarily for the size dependence of the transition temperature. For a further simplification we can assume Asv = Ass = A0 and (Δγss + Δγsv)/2 = ⟨Δγ⟩. Finally, we obtain

2⟨Δγ ⟩A 0 2⟨Δγ ⟩ A 0 B = T1/2 + Δs V ΔS 2⟨Δγ ⟩ 1 + Δs h

B + T1/2 = T1/2 B = T1/2

(3)

with Δs being the entropy density, V being the film volume, and h being the film thickness. It is important to note that eq 3 does not take into account explicitly the effect of surface stress, but we shall consider that both surface energy and surface stress contribute to the shift of T1/2.6 The best fit of eq 3 to the 25619

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the effect of film thickness on the spin transition temperature of 1. Nevertheless, other parameters (e.g., defects) that remain hidden in the experimental characterization of the samples might also play a role here. From this point of view, the observed rather small shift of T1/2 (ca. 3 K) and the tightly comparable shape of the SCO curves (Figure 1c) are very useful features, as they prove that the films are indeed very similar in all aspects, except their thickness. A higher shift of T1/2 can be obtained simply by increasing the surface to volume ratio of the objects (e.g., in small SCO nanoparticles16), but this will also inevitably increase the uncertainty in the comparison of the different-sized objects.

experimental data is shown in Figure 2a. To further improve the fit, we took into account the experimentally determined

4. CONCLUSIONS In summary, careful measurements on high-quality films of [Fe(HB(tz)3)2] give unambiguous evidence for the elevation of the spin transition temperature when reducing the film thickness. We show that this unexpected behavior is fully compatible with the nanothermodynamical theory of spin transitions by assuming an increase of the relevant surface energies when going from the low-spin to the high-spin phase (instead of the usual decrease). The particular anisotropy of the transformation strain in the oriented films appears as the origin of this behavior. This result calls for a modification of the paradigm of how the phase diagram of nanocrystalline materials (not only SCO) scales as a function of size and indicates the tight relationships among structural anisotropy, surface energy, and phase stability.

Figure 2. h−T phase diagram of oriented [Fe(HB(tz)3)2] thin films. Blue circles are the averaged experimental data. (a) The dashed line is the fit using eq 3, i.e., assuming the same surface area for each film. (b) The orange squares are fitted values using both eqs 3 and (4), i.e., taking into account the experimental surface roughness. The inset in part b shows the mean surface roughness vs film thickness extracted from the AFM topography images.

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surface roughness by assuming a linear variation of the surface area with the roughness:

A = A 0(1 + nR a)

(4)

S Supporting Information *

As shown in Figure 2b, considering the surface roughness of the films not only improves the fit but also allows one to understand the apparent nonmonotonous evolution of T1/2 below and above ca. 100 nm thickness. The fit parameters, n, ⟨Δγ⟩, and TB1/2 are summarized in Table 1. The obtained bulk

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b10124. AFM images used for the evaluation of film thickness and surface roughness, transition temperatures on heating and cooling, test of photothermal effects, and details of the theoretical study (PDF)

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Table 1. Fit Parameters Obtained Using eqs 3 and 4, with and without Taking into Account the Surface Roughness fit parameters

without roughness

with roughness

TB1/2 (K) ⟨Δγ⟩ (mJ/m2) n (nm−1)

336.6 ± 0.3 7.53 ± 0.9 0 (fixed)

335.9 ± 0.3 5.18 ± 0.5 0.87 ± 0.03

ASSOCIATED CONTENT

AUTHOR INFORMATION

Corresponding Authors

*G.M. e-mail: [email protected]. *A.B. e-mail: [email protected]. ORCID

Lionel Salmon: 0000-0002-8064-8960 Gábor Molnár: 0000-0001-6032-6393

transition temperature (ca. 336 K) is reasonably close to the values reported for powder and single crystal samples (331− 334 K).27 The key finding, however, is the positive value of ⟨Δγ⟩. This unusual increase of the (00l) surface energy (and surface stress) in the HS phase is perfectly in line with the crystallographic data, which indicates an unusual area contraction of the (00l) crystal planes in the HS state, despite the expansion of the interplanar spacing and of the overall lattice volume. Interestingly, this finding not only explains the upshift of the spin transition temperature for reduced film thicknesses but also corroborates the unexpected bending behavior of silicon microcantilevers when actuated with thin films of 1.32 As for the magnitude of the (00l) surface energy change between the two spin states (ca. 5 mJ/m2), it is comparable with those reported for polymorphic thin films of organic compounds.4 This model is seen to describe thus quite closely

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the Federal University of Toulouse Midi-Pyrénées through the project IDEX Emergence NEMSCOOP (ANR-11-IDEX-0002-02). The Ph.D. grants of S.R., M.D.M.J., and A.C.B. were financed, respectively, by the MESR, the CONACYT (no. 382038), and the Région Occitanie (no. 15050123).

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