domain: chim phys

text:
Léo Bossard-Giannesini

Luis Cardenas

Hervé Cruguel

Aude Demessence

David Loffreda
email: david.loffreda@ens-lyon.fr

Olivier Pluchery
email: olivier.pluchery@insp.jussieu.fr

How far the Chemistry of the Self-Assembly Monolayers on Gold Surfaces Affects their Work Function?

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Alkanethiols represent a class of ideal molecules for building robust and reproducible functionalized gold surfaces. Interestingly, self-assembled monolayers (SAM) enable adding new functionalities to a gold surface for building biosensors, 1,2 modulating the surface electric properties, [3][4][5][6][7] and opening new perspectives for molecular devices. 8,9 Several reviews report on the numerous studies devoted to the morphology of SAMs on various metals. [10][11][12] Among the interesting molecules, dodecanethiol has been investigated in details and the corresponding SAMs arrange into a brush-like structure with the sulfur moiety strongly bound to gold. [13][14][15][16] This SAM is terminated by the methyl group, which makes the coated metallic surface fairly passive towards further reactions and other functionalizations. Other tail groups than CH3 were also studied, such as carboxy, alkoxy, amino or thio groups [17][18][19][20] or fluorinated tail groups. [21][22][23] Very active debates have appeared in the literature related to the change of this substituent and the subsequent modulation of the electrostatic properties of metallic surfaces. [24][25][26][27][28] Among these properties, the work function (WF) is the most emblematic one. It is defined for a surface as the required energy for extracting one electron from the Fermi level of the metal to the vacuum level, far from the surface. Such definition is adequate for ideal planar surfaces of homogeneous materials. 29,30 In molecular science, it is a concept similar to the ionization energy (IE) (see for example, critical comparison of WF and IE in Ref 31 ). IE and WF are playing central roles in the understanding of many fundamental mechanisms, in terms of chemical reactivity, photocatalysis, or nanoelectronics. Measuring values for the WF of functionalized surfaces has been carried out with various techniques: Ultraviolet Photoelectron Spectroscopy (UPS) under vacuum conditions, Kelvin Probe and more recently, Kelvin Probe Force Microscopy (KPFM) in atmosphere or in vacuum. 21,22,32,33 Knowing the WF is critical for devices, it has also been measured in various contexts including electric characterization of barrier heights in junctions, such as metal-organic interfaces, 8,9,21 lightemitting devices (LED), 6 or field emission guns. 34 For example, in the case of the most widely studied SAM, such as dodecanethiol on Au(111), the effective WF was measured mostly with UPS between 3.9 and 4.1 eV. 5,34,35 Debates in the literature call for a rationale of the WF predictions. In this context, it is tempting to evaluate the WF of functionalized surfaces by considering the dipole moment of the free molecules. It is known that the WF of a surface increases when it is covered with a sheet of inwards dipoles and decreases with outwards dipoles. 36,37 Let's denote 𝜇 ! the z-component of the electrostatic dipole perpendicular to the surface, the Helmholtz formula expresses the modification of the WF as follows:

∆𝑊𝐹 = - 𝜇 𝑧 𝜀 0 𝐴 = - 𝑞•𝑑 𝜀 0 𝐴 (1)
where A is the area occupied by one molecule, ±𝑞 corresponds to two opposite charges positioned at a distance d from each other. This electrostatic approach is discussed in depth by Heimel. 36,37 Thus, we can consider a realistic SAM made of molecules organized as a square lattice of parameter a = 0.2 nm. If each molecule bears a dipole of -1.0 D which corresponds to 𝑞 = ± 𝑒 10 ⁄ , and a charge separation 𝑑 = 0.2 nm, this SAM would generate a very large WF change of +9.0 eV. 37 This pure electrostatic approach predicts ∆𝑊𝐹 values that are ten times larger than the typical experimental values, and is probably too simplified. Other contributions play a major role, such as the highly polar molecule-surface bond, 24,38 or a depolarization factor due to the compression of molecules in monolayers which decreases the impact of the initial molecular dipole. 39 More elaborated theoretical models describing the chemical bonding have been reported in the literature based on density functional theory (DFT) and quantum mechanics. Several approaches were proposed either by calculating the resulting dipole of a floating molecular sheet 8 or by developing a more realistic SAM in interaction with gold. 24,25,36 More recently, Van der Waals interactions between the molecular chains were also included in the DFT models with various functions. 20,[40][41][42][43][44] The considered supercells aiming to describe the SAM on gold are usually rather small in order to significantly reduce the computational cost, thus restricting the exploration of the complex relative conformations and orientations of the coadsorbed molecules at long range. In addition, in experimental conditions the presence of contaminants adds a supplemental and hardly incontrollable dipole contribution that blurs the WF values. 3 It results that the effective WF modification of a thiol-covered gold surface may depend on the following factors: (1) the permanent dipole of the isolated molecule, that can be adjusted by modifying the tail group, (2) the molecule chain length, (3) the surface packing density (4) the polarity of the gold-sulfur bond and (5) the presence of polar contaminants.

The aim of the present work is to provide a detailed study of the modification of the WF of reconstructed Au(111) gold surfaces when they are functionalized by alkylthiol molecules with different chemical terminations. The SAM formation is investigated with two experimental techniques and a set of state-of-the-art DFT calculations allowing the exploration of complex coadsorption motifs with large supercells and a large number of molecules. Four terminal groups are addressed: methyl, carboxyl, amino, and thio (see Figure 1). We also discuss the effect of the chain length. We show that the WF is systematically decreased compared to bare gold, even with molecules exhibiting a strong inward dipole in their free state.

Experimental

Gold surface preparation. Evaporated gold. Gold surfaces are obtained by thermally evaporating 200 nm of pure gold on a glass substrate (microscope slide from Menzel Gläser) without any adhesion layer, under vacuum (3 x 10 -6 mbar). The glass substrate was first degreased with acetone and ethanol. The resulting evaporated gold surface was rinsed with absolute ethanol (99.9 %) and dried with a nitrogen flow. Reconstructed gold. Evaporated gold surfaces were reconstructed under a propane flame during 30 s. The control of the flaming temperature is important and is achieved by ensuring a homogeneous orange color of the surface in a completely dark room. This surface was then cooled down and rinsed copiously with absolute ethanol during 30 s. A drop of ethanol was left for protecting the surface from air contamination before functionalization. For the study of the reconstructed gold, the surface was dried under a nitrogen flow. This experimental protocol is well documented and is known to produce facets of (111) orientation on gold. 19,[START_REF] Kolb | Nanoscale Probes of the Solid/Liquid Interface[END_REF][START_REF] Vaz-Domínguez | [END_REF] Functionalization of the gold surfaces. Reconstructed gold surfaces were functionalized with thiol molecules in order to form the SAMs. Four types of thiols are used: 8-amino-1octanethiol (AOT) (98.5 %), 16-mercaptohexadecanoic acid (MHDA) (99 %), 1,6-hexanedithiol (HDT) (97 %) and dodecanethiol (DDT) (98 %) (Figure 1-a). All compounds were purchased from Sigma-Aldrich. SAMs were deposited on freshly prepared gold surfaces by covering the surface with the solutions. Thiol solutions were prepared in absolute ethanol with a concentration of 10 mM for AOT, MHDA and DDT, and a concentration of 1 mM for HDT. The immersion time is of 90 min for AOT, MHDA and DDT, and 30 s for HDT. In the case of HDT, this short immersion time was selected to avoid the formation of disulfide bridges between tail groups. Notice that if disulfide bridges are formed, they can be detected with STM as parallel linear motifs in the SAM. 19 These parameters enabled a complete saturation of the surface by the molecules. Surfaces were then dried under a nitrogen flow. Kelvin Probe Force Microscope.

Topography and contact potential differences (CPD) were measured simultaneously with a commercial Kelvin probe force microscope (KPFM, Multimode 8, Bruker) in a dual pass mode.

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The CPD channel is acquired within the AM (amplitude modulation) mode and with an accuracy of 5 mV. Topography is acquired with the Peakforce® mode which allows controlling the interacting force with the surface and preserves the tip sharpness. KPFM was recorded at room temperature in a humidity controlled environment where the relative humidity (RH) was adjusted between 3 % and 40 % (room atmosphere). RH was controlled by flowing ultrapure nitrogen gas into the KPFM sealed compartment. Ultraviolet-Photoemission Spectroscopy.

UPS analysis was carried out in an integrated commercial ultrahigh vacuum photoemission system Axis ULTRA DLD spectrometer. UPS spectra were obtained using a helium ionization lamp with two resonances lines, He I at 21.2 eV and He II at 40.8 eV. To calibrate the work function of the analyzer, a negative bias potential of 9.1 V was applied to the sample. Prior to measurements, an Ag foil was cleaned by argon ion sputtering. The bias was optimized to shift the Ag spectrum in the linear region of the analyzer (0-10 eV kinetic energy). 47 A work function of 4.2 eV was obtained, in agreement with the literature value of 4.26 eV. 48,49 The WF on each surface was calculated using the energy cutoff obtained from the first derivative of the UPS spectrum (φ = Ecutoff ± 21.2 eV) .

WF calculations with DFT modelling.

In this study, Density Functional Theory calculations were performed with the VASP 5.4 package. [50][51][52] The electronic exchange-correlation GGA PBE functional 53 was considered in combination with the semiempirical dispersion-correction method DFT-D3 in its zero-damping formalism. 54 The interactions between electrons and ion cores were described by potentials generated by the Projector Augmented-Wave method (PAW potentials, 11 valence electrons per gold atom). 55 Valence electrons were expanded in plane waves with a kinetic cutoff energy of 400 eV (in the range 7263-9315 valence electrons depending on the molecule). A large p(9x9) supercell of a Au(111) slab composed of 7 atomic planes was used (corresponding to a hexagonal box with lattice vectors of lengths 25.92×25.92×49 Å 3 ) with adsorption of various SAM on one side of the slab (nonsymmetric adsorption). A large vacuum space along z-axis (in the range 12.8-27.3 Å according to the molecule) was described in order to avoid spurious interactions between the adsorbate and the other side of the periodic metallic slab. Five different molecules (see Figure 1-b) were adsorbed on the Au(111) slab through the best adsorption structure (top site through S atom of the SH moiety). The thiol molecules were adsorbed molecularly on the gold surface since a previous theoretical study showed that dissociative adsorption of long alkyl chain thiols (leading to thiolates) is not favored. 15 SAMs of these molecules were generated by co-adsorbing 27 molecules in the p(9x9) supercell leading to a (√3x√3)-R30°-27thiol adlayer motif (see Figure 6-a). The chosen surface coverage of 1/3 ML (monolayer) corresponds to adlayers close to saturation, as depicted in Figure 6 for the SAM of MHDA and Figure 7 for other SAMs. Due to the selected large supercell, the Brillouin zone sampling in reciprocal space was restricted to the Γ point ensuring a good convergence of the total electronic energy. For Au bulk, the grid of Brillouin zone used for determining accurate total electronic energy was 39×39×39 k-points, thus giving an optimal lattice parameter a0 of 4.0727 Å (in close agreement with measurements: 4.0783 Å) and cohesion energy of -3.69 eV/at. (also in fair agreement with experiments, -3.81 eV/at.). For dealing with the partial occupancies around the Fermi level, a Methfessel-Paxton smearing was used with σ = 0.2 eV. The geometry optimizations were completed by relaxing entirely the degrees of freedom of the adlayers and of the metallic planes, with a convergence criterion of 10 -6 eV for the total electronic energy, until each residual force on nuclei was less than ±0.01 eV.Å -1 . The calculations of the work function were based on the predictions of the electrostatic potential. For each SAM on Au(111), the integrated potential in the (x,y) plane was predicted by single-point energy calculations on the basis of previously optimized structures and plotted along the z axis, in much more accurate computational conditions (kinetic energy cutoff of 500 eV and a vacuum space systematically increased by 10 Å along the z axis to help the convergence of the vacuum level). The WF (positive value) was then derived by the difference between the position of the vacuum (positive value) and the Fermi level (negative value) (see Table 2). The systematic error of the computed work function has been estimated by changing the thickness of the gold slab from 3 to 7 layers and has a value ±31 meV. b) the DFT calculations. Four types of molecules are used differing by their end group: thiol moiety for hexanedithiol (HDT) and butanedithiol (BDT), amine moiety for aminooctanetiol (AOT) and aminobutanethiol (ABT), methyl moiety for dodecanethiol (DDT) and pentanethiol (PT), or carboxylic moiety for mercaptoundecanoic acid (MHDA). and mercaptopropanoic acid (MPA). The resulting permanent dipole of the isolated molecule is also indicated and expressed in debye, as calculated with the Hückel approach (in blue when positive and in red when negative).

Results

CPD of bare and functionalized gold

First, the morphology of the gold surfaces before the functionalization was investigated. With KPFM, the evaporated gold surfaces exhibit a uniform morphology (Figure 2-a) but their potential is not homogeneous as shown in Figure 2-b. On 2x2 µm² images, important drops of potentials of 100 mV were observed due to adventitious adsorbates and local surface defects. In the following, systematic flame annealing was performed in air, followed by a rinsing with ultrapure ethanol, in order to yield a reconstructed surface. With this method we obtained morphologically flat surfaces and large terraces of This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins 400 nm, with a homogeneous potential (< 5 mV variations) and stronger contrasts (50 mV) at the step edges as visible in Figure 2-c) andd). After SAM formation with the four molecules, the terraces are preserved. Impact of the relative humidity on the WF. After the SAM formation, the evolution of the CPD as a function of RH was monitored. KPFM images were acquired line by line and in Figure 3, the average of the CPD values over each line is plotted as a function of time (grey line), simultaneously with the RH (green dotted line). We can distinguish three regions: region I is just after the SAM formation, region II is 2 h after the SAM formation and region III, 24 h later. During these 24 h, the sample was in contact with the ambient atmosphere. These three time-delays correspond roughly to three typical durations when the samples are manipulated in atmosphere: delay I is the quickest delay between the end of the chemical preparation and the first CPD measurement. Delay II, is the duration of an average KPFM experiment session for one sample and delay III is considered as the stabilized surface. In Figure 3 the CPD of the reconstructed gold surface functionalized with a SAM of HDT is presented. We can observe two phenomena. First, the potential evolves towards more positive values, with time and with the ambient atmosphere: from -0.33 V to +0.05 V. Then the introduction of N2 and the decrease of humidity to 3 % do not restores the potential to the starting value, but it slows down the evolution. Finally, a stability is reached after 24 h (part III). From the deposition of the SAM to the stability after 24 h, the potential has increased by 0.38 V. The four SAM studied in this article present a similar behavior with albeit different kinetics and different amplitudes (see a summary in Figure 4). 5 where e is the elementary charge (positive value). In our experiment the bias is applied to the surface, therefore an increase of the WF of the sample results in an increase of the measured CPD. 32,56 The conversion of CPDs into an absolute value of the WF requires a calibration, which is far from simple. 32 In the present case, calibration is carried out with a reference sample made of aluminum, covered with an alumina thin oxide. This surface exhibits a stable WF in air of 4.55 eV. 57 Results are presented in Figure 4. The calibration was achieved by systematically measuring the aluminum surface before and just after recording the images of the functionalized surface of interest. This allows detecting the possible change of the WF of the tip during an experiment. The largest variation measured was 140 mV. Therefore, the uncertainty of the measured WF in this study is ±70 meV. The surface of evaporated gold evolves from 4.72 eV to 4.80 eV in 24 h. The flamed gold surface exhibits a WF of 4.87 eV and the time evolution was not monitored. For the MHDA SAM, the WF increases regularly with time of about 0.30 eV within a day in air. Gold surfaces functionalized with AOT and HDT evolves in a very similar way from an initial value of 4.28 eV and a slow increase to 4.74 and 4.91 eV, respectively. This is not the object of the present work to discuss the reason of these evolutions strongly related to the water molecules. 58 Finally, the DDT SAM exhibits the lowest WF among the SAMs in our study, at 3.90 eV. This value agrees well with those reported in literature. 5, 34, Please do not adjust margins Please do not adjust margins These data show that the WF of the freshly prepared SAMs (delay I) provide the most meaningful values. They are summarized in Table 1 and will be discussed and compared with those obtained with UPS and DFT.

UPS measurements and WF of SAM surfaces.

The WF measurements with photoelectron spectroscopy were carried with a special attention to some specific requirements. First of all, the incident photon energy has to exceed the WF of the sample. The WF of the analyzer and the sample were compensated by applying a constant potential during the measurement. In the case of a metal-semiconductor contact, the Fermi level of both surfaces are aligned under thermodynamic equilibrium conditions. Therefore, according to the Schottky-Mott principle, a redistribution of the charges takes place, which results into the formation of a dipole Δ at the interface. 59,60 UPS probes an area of the surface of ca. 1 mm of diameter and gives therefore access to the spatially averaged value of the WF. Figure 5 displays the two spectral regions of interest in the UPS spectra: the first region (a) is dominated by the secondary electrons (SEC) near the cutoff energy ECutoff. After their extraction from the surface, these photoelectrons are emitted with a kinetic energy between 1 and 10 eV, which is their remaining energy after multiple inelastic scattering events. The electrons probe a depth of a few nanometers below the surface. Nevertheless, the secondary electrons retain the initial information (escape energy), and eventually provides the value of WF. In contrast, Figure 5-b shows the spectral region of the primary photoelectrons, which do not undergo inelastic scattering. They represent, in a first approximation, the density of states of the material, and therefore their energies correspond to the electron binding energies. To facilitate the calibration of the energy scale and of the Fermi level, a gold foil was cleaned through a sequence of ion sputtering with ionic argon energy of 1KeV. The UPS spectrum of this pristine gold foil (Figure 5-a, bottom spectrum) shows the excited electronic states observed as a hybridized flat band related to the d and s/p states located between EF and ≈ 1.8 eV. 61 Subsequently, the d electronic states of polycrystalline gold are located between ≈ 2 eV and 8.5 eV. The WF was calculated with the assistance of the cutoff energy (Ecutoff) obtained from the first derivative of the SEC spectrum. The WF for pristine gold is 5.1 eV, in good agreement with the literature value. 59 Therefore, based on Fermi level alignment between Au and SAMs, one can use the reminiscent states (including EF) of gold observed in the different spectra to calibrate each of them. This is particularly visible for the localized state at 6 V and the EF = 0 eV, which are observed in every spectrum, excepted for MHDA. However, the broad feature observed on MHDA around 6 eV fit with both states that are visible on gold pristine at ≈6 eV and ≈4.3 eV. The emergence of these Au electronic states depends on the homogeneity and coverage rate of the Au surface by SAMs. This calibration methodology validates the visible interface dipole Δ in the Ecutoff region of each UPS spectra (SAMs/Au), which varies between ±0.4 eV and ±0.9 eV forming an interface dipole barrier. 62 It plays an essential role in the direction of electron transfer in the SAM-Au junction, and thus Δ can be used as a predictive parameter. 63 The interface dipole Δ is simply related to the change of the WF, ΔWF upon adsorption by ΔWF = -Δ. The results are reported in Table 1.

DFT calculations of the WF of SAM surfaces

To predict the variations of the WF from a theoretical approach, the SAMs of five different molecules were optimized on the basis of DFT calculations with the most accurate computational and modeling conditions: a large supercell, thick metallic slabs, a large vacuum space, a large number of co-adsorbates, high coverage close to saturation, including van der Waals weak interactions, using tight convergence criteria. MHDA. From a computational standpoint, the most elaborated SAM of our study is this 27 MHDA that are coadsorbed molecules on Au(111) (see Figure 6 for different views and definitions). In our models, the thickness of the organic coating is almost equivalent to the thickness of the metallic slabs. The adlayer is composed of an initially perfectly distributed set of 27 MHDA molecules forming a regular checkerboard with a (√3x√3)-R30° adlayer motif (see Figure 6-a), with identical lateral orientations (eclipsed conformation). After geometry optimization, the SAM forms a wavy comb of MHDA chains (see top view in Figure 6-b). From lateral standpoints, the SAM forms either a bent comb (Figure 6-c) or a more regular comb of almost eclipsed MHDA molecules (Figure 6-d). The SAM (comb shape) is significantly inclined with respect to Au(111) as shown in Figure 6-d in agreement with previous DFT results. 20,41 The carboxy moieties at the tail of the MHDA molecules change their initial parallel orientations during the geometry optimization, to form a discontinuous network of stabilizing hydrogen bonds between the carboxylic acids (in the range 1.75-2.45 Å) as depicted in Figure 6-e. This network constrains the bound MHDA molecules at their tails to keep an eclipsed conformation of their long hexadecyl chains. The adsorption energy per MHDA, including Van der Waals interactions, is quite strong (-2.186 eV/molec) and is largely dominated by Van der Waals interactions, also called dispersion (105 % of the covalent bonding contribution) due to the hexadecyl C16 long aliphatic chains. The increase of these long-range interactions with the length of the aliphatic chain has been previously detailed in the literature, with lengths up to C12 alkylthiol on Au111. 41 The related WF of 4.99 eV (see Table 2) is shifted to lower values with respect to the predicted WF for clean Au(111) surface (5.26 eV). 49,64 MPA. By considering a much shorter chain (C4 instead of C16), the corresponding SAM of MPA was modelled and optimized in equivalent conditions with respect to MHDA (see Figure 8-d, h). The adsorption strength is much weaker (-1.349 eV/molec) and less dominated by dispersion energy (86 %), although the presence of hydrogen bonds between carboxy moieties is also evidenced (notice how the almost eclipsed conformation of the MPA molecules is preserved) in agreement with Ref. 41 The decrease of the chain length also affects the WF which is shifted to lower values (4.78 eV) compared to the SAM of MHDA.

ABT. The substitution of the carboxy moieties by amino groups was then investigated by modeling the SAM of ABT on Au(111) (see Figure 8-c, g). Such a chemical change produced a concomitant decrease of the adsorption strength (-1.175 eV/molec, more dominated by dispersion: 93 %) and of the WF (4.76 eV). The absence of hydrogen bonds in this SAM offers more flexibility to optimize the relative conformation of ABT molecules thus leading to a less eclipsed configuration. PT. At the opposite, the substitution of carboxy groups by methyl moieties gives rise to a SAM of PT molecules with a weaker adsorption strength (-1.143 eV/molec) but a larger WF (4.85 eV). BDT. When a symmetric dithiol molecule (BDT) is used to build a SAM on Au(111), the loss of adsorption strength is more moderate (-1.224 eV/molec), while the WF is the largest one for the series of C4-chain molecules (4.87 eV). In the latter two cases (PT and BDT), no hydrogen bond network is observed likewise the SAM of ABT. This explains the more regular relative conformations of the molecules self-organized into star-like lattice.

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Discussion

The Figure 8 presents a graphical summary of the experimental (a) and theoretical results (b) and will be the base of our discussion.

Absolute values of the WF. This shift has to be linked to the effects of contaminants on surfaces, namely water molecules. In vacuum the WF of Au( 111) is measured at 5.33 ± 0.06 eV 49 and our calculations also predicts a WF at 5.26 eV (see Table 2). Heimel et al. have demonstrated that sparse dipolar molecules on a surface, such as water contamination, add an important electrostatic contribution to the overall potential energy and cause the WF to be shifted. 37 Please do not adjust margins Please do not adjust margins +5.83 eV) 17 . On the same line, the HDT monolayer should induce no WF change or at least a very negligible one compared to AOT. However, our experiments show that HDT and AOT exhibit the same DWF of -0.59 eV. These results reveal certainly the dominating influence of other parameters than the dipole moment of the free molecule. Actually some key parameters can be listed: the polarization of Au-S bond 24,38 , the length of the molecular chain, the surface density of the molecules within the SAM, and the existence of hydrogen bonding between the tail-groups. 20 Therefore it should be concluded that the dipole moment of the free molecule has a very limited electrostatic influence on the final work function of the functionalized surface. Helmholtz equation is not wrong, but to be predictive it has to include the calculation of the resulting dipole of the adsorbed molecules and not just the dipole moment of the free molecule. Insightful results were published in the group of Zharnikov where they changed the molecular dipole of their SAM, and subsequently its WF, by either embedding different dipolar esters inside the chain molecules 26,28 or mixing upwards and downwards dipoles. 33 With this approach, they could change the molecular dipoles without modifying too much the metal-molecule bond and they confirm that the final WF did not vary much.

Comparison between experiments and theory for MHDA.

To gain insight about these shifts, the case of MHDA is interesting since this functionalization was considered with our three approaches. All these approaches conclude to a decrease of the WF yielding very close values: -0.40±0.1 eV with UPS, -0.30±0.05 eV with KPFM and -0.27±0.03 eV with DFT. This agreement validates our DFT approach which will thus be useful for analyzing the various contributions to the WF. Due to the computational cost, similar DFT calculations for the other three molecules investigated experimentally (HDT, AOT et DDT) were not carried out. This is why shorter chain lengths were investigated in our DFT models compared to the molecules used in experiments, although the anchoring groups and the tail groups were kept identical. Effect of the chain length. For a given tail group, the chain length has been varied from four carbons (MPA, DWF =-0.48 eV) to 16 carbons (MHDA, DWF =-0.27 eV): the WF is shifted of 0.21 eV. This agrees with the early results from Evans and Ulman. 65 It shows the longer the chain length, the smaller the WF modification. Actually, this trend is confirmed by the experiments as shown on Figure 8-a show that longer chains induce an increase of the WF 44 in contrast with our calculations at coverages close to saturation. At higher coverage, the lateral interactions between molecules inside a SAM are stronger. This mutual interaction drastically affects the WF. Indeed, a recent theoretical study showed that the interchain energy has a leading impact on the stability of the SAM as a function of the chain length. 41 The important result here, is that changing the molecular chain length from C6 to C16 accounts for a significant WF decrease of ca 0.25 eV, much larger than the systematic experimental uncertainty. The case of DDT. Figure 8-a shows that DDT behaves very differently in some experiments. It exhibits a very strong DWF of -0.97 eV in agreement with the available literature. 5,34,35 The specificity of DDT comes from synergistic effects: a higher compacity of the SAM leading to higher coverage, and the absence of a hydrogen-bond network between the tail groups.

According to our DFT results, both concomitant effects lead to a decrease of the WF. Since they do not take place in the SAM of DDT, this explains why its WF change reaches -1.0 eV. The nature of the tail group. Four different tail groups were considered in our DFT models with the same chain length of C4 (Figure 8-b). BDT is a non-polar molecule and exhibits the smallest WF decrease (-0.38 eV). ABT and MPA are the most polar ones and they induce the largest DWF (-0.49 eV and -0.48 eV, respectively). This confirms that the dipole moment of the molecule plays a role in the final value of the WF. 23 It corresponds to a contribution of -0.1 eV which is smaller than the contribution of the chain length. This 0.1 eV is in sharp contrast with the values of the electric dipole of the free molecules that were computed to +0.49 D and -5.25 D for ABT and MPA, respectively. We can conclude here that the nature and the polarity of the tail group has a rather limited influence on the final WF change when SAMs are elaborated on Au(111), which agrees with the conclusions from Ref. 43 on SAMs of bipyrimidine and from Ref. 66 where the tail dipolar group is flipped thanks to the azobenzene photoisomer. Impact of the hydrogen bond network on the WF of the SAM. Finally, in the case of the MHDA molecule, we have shown that the long C16 chains provides flexibility to the COOH tail groups which eventually assemble through a complex hydrogen bond network (see Figure 6-e). Those additional bonds induce disordering in the SAM, which leads to a spreading the electric charge, and to a reduction of the surface molecular dipole. One can conclude that the charge distribution may be more uniform, thus reducing the resulting dipole moment. The strong impact of the disordering inside the SAM on the WF of the surface is also confirmed by recent publications. 22,67

Conclusions

We have investigated how the SAMs of families of thiol molecules affect the WF of Au(111), by using both theoretical and experimental techniques. First, the WF of the clean reconstructed Au(111) strongly depends on the environment; it is predicted at 5.26 eV in vacuum and detected at 5.1 eV by UPS in an UHV chamber (3 x 10 -6 mbar) and at 4.87 eV in air. The adsorption of a SAM with a thiol anchoring group systematically induces a significant decrease of the WF from 0.97 eV to 0.30 eV, which is hardly predictable based on the value of the dipolar moment of the isolated molecule. Even the MHDA molecule, which has a strong inwards dipole, induces a decrease of the WF of 0.30 eV (calculated at 0.27 eV by DFT). From our results, we show that the largest impact on the WF change is due to the chain length (contribution of 0.2 eV), where the longer the This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins chain, the smaller the WF modification. Then, the nature of the tail group contributes at a lesser extent, ~0.1 eV, with the largest influence assigned to the carboxy moiety, then amino, methyl and thio groups. We also demonstrated the strong influence of the hydrogen bonding network between the tail groups at the surface of the SAM, as demonstrated by our DFT calculations for the case of MHDA. These hydrogen bond networks tend to smear out the charge localization and reduce the dipole contribution. This effect results in the lowest value of DWF =-0.30 eV for MHDA (dense network of hydrogen bonding between the COOH moieties) and DWF =-0.97 eV for DDT (no hydrogen bonding).

Figure 1 .

 1 Figure1. Molecules used for the SAM formation on gold surfaces for a) the experiment and b) the DFT calculations. Four types of molecules are used differing by their end group: thiol moiety for hexanedithiol (HDT) and butanedithiol (BDT), amine moiety for aminooctanetiol (AOT) and aminobutanethiol (ABT), methyl moiety for dodecanethiol (DDT) and pentanethiol (PT), or carboxylic moiety for mercaptoundecanoic acid (MHDA). and mercaptopropanoic acid (MPA). The resulting permanent dipole of the isolated molecule is also indicated and expressed in debye, as calculated with the Hückel approach (in blue when positive and in red when negative).

Figure 2 :

 2 Figure 2: AFM-KPFM images (2x2 µm 2 ) of bare gold surfaces: evaporated gold with a) topography and b) CPD map, and reconstructed gold with c) topography and d) CPD map. The image is recorded line by line and thus allows a potential measurement as a function of time.

Figure 3 :

 3 Figure 3: evolution of the measured potential (grey curve) of a HDT SAM on gold as a function of time. The relative humidity measured in real time in the box is also shown (green dotted line). Corresponding introduction of N2 or air in the box is also indicated. CPD of each SAM and calibration of the WF. CPD values are directly obtained from the KPFM images. CPDs are relative values, defined by 𝑉 ()* = 1 𝑒 ⁄ 4𝑊𝐹 +,-./0 -𝑊𝐹 12.5 where e is the elementary charge (positive value). In our experiment the bias is applied to the surface, therefore an increase of the WF of the sample results in an increase of the measured CPD.32,56 The conversion of CPDs into an absolute value of the WF requires a calibration, which is far from simple.32 In the present case, calibration is carried out with a reference sample made of aluminum, covered with an alumina thin oxide. This surface exhibits a stable WF in air of 4.55 eV.57 Results are presented in Figure4. The calibration was achieved by systematically measuring the aluminum surface before and just after recording the images of the functionalized surface of interest. This allows detecting the possible change of the WF of the tip during an experiment. The largest variation measured was 140 mV. Therefore, the uncertainty of the measured WF in this study is ±70 meV. The surface of evaporated gold evolves from 4.72 eV to 4.80 eV in 24 h. The flamed gold surface exhibits a WF of 4.87 eV and the time evolution was not monitored. For the MHDA SAM, the WF increases regularly with time of about 0.30 eV within a day in air. Gold surfaces functionalized with AOT and HDT evolves in a very similar way from an initial value of 4.28 eV and a slow increase to 4.74 and 4.91 eV, respectively. This is not the object of the present work to discuss the reason of these evolutions strongly related to the water molecules.58 Finally, the DDT SAM exhibits the lowest WF among the SAMs in our study, at 3.90 eV. This value agrees well with those reported in literature.5, 34,

Figure 4 :

 4 Figure 4: evolution of the WF of functionalized gold surfaces as a function of the contact time with the ambient atmosphere. WF values were obtained with calibrated KPFM measurements. Measures I are carried out in a controlled (dried) atmosphere or 3% RH. For measures II and III the samples have been exposed to air for at least 1 hour.

Figure 5 :

 5 Figure 5: UPS spectra of (a) SEC and (b) primary band regions corresponding to Au, MHDA, AOT, DDT, and HDT, respectively.

Figure 6 :

 6 Figure 6: Different views of the optimized structure for a SAM of MHDA molecules adsorbed on Au(111) from DFT calculations : (a) top view of the Au(111) slab composed of 7 atomic layers along z axis and a (9x9) supercell in the (x,y) plane (light blue dotted lines). 27 MHDA molecules are coadsorbed in this supercell leading to a (√3x√3)-R30°-27 MHDA adlayer motif (corresponding to a molecule surface density of 4.64 molec/nm²) with a hexagonal distribution of adsorbates as shown by the red hexagon (the adsorption sites being mentioned by the positions of the SH moieties bound to Au through S atoms on top sites); (b) top view of the SAM that forms a "wavy comb" through the organization of MHDA adsorption structures; (c) lateral view of the SAM along the x axis that forms a "bent comb" through the curvature of the hexadecyl chains; (d) lateral view along y axis that forms a more "regular comb" with linear and almost eclipsed hexadecyl chains; (e) top view of the SAM by masking Au(111) and by indicating the hydrogen bonds registered between COOH moieties (distances in . Au atoms are depicted with golden balls, S atoms with yellow balls, C and H atoms belonging to the hexadecyl aliphatic chains with gray and white sticks, respectively, C and H atoms of the carboxy moieties with black and white balls, respectively, O atoms with red balls.

Figure 7 :Table 2 .

 72 Figure 7: Different views obtained from DFT calculations of the optimized structures of four SAMs adsorbed on Au(111): lateral views of 27 coadsorbed (a) pentanethiol (PT), (b) butanedithiol (BDT), (c) aminobutanethiol (ABT), (d) mercaptopropanoic acid (MPA), and top views (e)-(h) of these SAM, respectively. The supercell, coverage and thickness of the metallic slab are identical to the SAM of MHDA exposed in Figure 6. Au atoms are depicted with golden balls, S atoms with yellow balls, C atoms with gray sticks, O atoms with red balls, N atoms with blue balls, H atoms with white balls.

Figure 8 .

 8 a) compares the WF values obtained with KPFM (colored vertical lines) and with UPS (black horizontal rods). The clean Au(111) reconstructed surface is measured at 4.87 eV with KPFM and 5.1 eV with UPS. KPFM values are systematically lower than those of UPS of ~0.2 eV.

Figure 8 .

 8 Figure 8. Work function of gold surfaces and its modification DWF upon functionalization: a) experimental values obtained from the KPFM measurements (colored vertical lines) and from UPS measurements (black horizontal rods). b) data extracted from the DFT calculations with the molecules The numerical values for DWF are indicated for each molecule on the graph.Castanon et al. worked with a KPFM equipped with platinum tips and show that the presence of humidity may sometimes induce an increase of the WF of the tip of 0.6 eV, and a subsequent equivalent decrease of the WF of the sample.58 Therefore when a sample is analyzed in air with KPFM, a lowering of the WF is expected compared to vacuum (10 -8 mbar for UPS). We will continue our discussion by focusing on variations of WF (DWF). They are in good agreement for the two experimental techniques, if we exclude the molecule of DDT (see the discussion of this case below). The numerical values for these shifts in KPFM are written on Figure8. As a partial conclusion, these data confirm that the ideally cleaned Au(111) surface has a WF of 5.26 eV. Under vacuum with a possible thin adlayer of physisorbed water, UPS detects a WF for Au(111) of 5.1 eV. Finally, when exposed to air, KPFM measures a WF of 4.87 eV for this same surface. Is there a relation between the molecular dipole moment of the free molecule and the WF? From the Helmholtz equation(1) and from the values of the dipoles of the free molecules, one could in principle predict the values for DWF. For example, the monolayer of MHDA should exhibit an increase of the final WF due to the strong inward dipole of -5.39 D (see Figure1-a). However, DWF of MHDA is negative with our three approaches, despite contradictory results from Lee et al. (WF measured at

Experimental values of the WF of SAM on reconstructed Au(111) KPFM UPS

	Molecule	DWF	WF	DWF	WF
		(V)	(eV)	(V)	(eV)
	Flamed gold	0	4.87	0	5.1
	HDT	-0.59	4.28	-0.90	4.2
	Hexanedithiol
	AOT	-0.59	4.28	-0.70	4.4
	Amino-octanethiol
	DDT	-0.97	3.90	-0.60	4.5
	Dodecanethiol
	MHDA	-0.30	4.57	-0.40	4.7
	Mercapto-hexadecanoic
	acid

6 | J. Name., 2012, 00, 1-3

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Table 1 .

 1 Experimental values for the WF obtained with KPFM and with UPS. KPFM values are those obtained immediately after surface preparation (in the time delay I at 3 % RH). The values of DWF correspond to the decrease of the WF after the gold surface is functionalized by one of the four molecules. The KPFM values of the WF are calculated using the reference surface of aluminum (WF = 4.55 eV). The WF values obtained with UPS are acquired in vacuum.

  , if DDT is excluded once again. The four molecules are represented in the order of increasing chain length from C6 to C16 and DWF decreases. The WF modification is of 0.29 eV here. With a longer chain, the molecule can accommodate more easily the local charges and decrease the resulting electrostatic dipole. Interestingly, calculations from Tong et al. at a lower molecule coverage,

Acknowledgements

The authors thank PSMN in Lyon and GENCI national resources (IDRIS, CINES, TGCC, project 609) for CPU time and assistance. The authors thank the CPER/SYSPROD 2015-2022 project (N°2019-AURA-P5B) and AXELERA Pôle de Compétitivité (PSMN Data Center). KPFM acquisitions were carried out at the Advance Atomic Force Microscopy platform (advAFM) of Sorbonne Université.

ARTICLE

This journal is © The Royal Society of Chemistry 20xx J. Name., 2019, 00, 1-3 | 8

Conflicts of interest

The authors do not declare any conflict of interest.


Rationale 2. The case for PIDINSTs 2.1 Technical approaches to PIDINST implementation 2.2 The role of Current Research Information Systems (CRIS) for the implementation of PIDs 3. Issues around risks and trust regarding the implementation of PIDINSTs 3.1 'Technical' vs admin-oriented PIDs 3.2 Community engagement 3.3 Technical divergence 3.4 Overload of PID infrastructure providers 4.

Rationale

This case study aims to explore the challenges faced and the opportunities offered by the gradual implementation of emerging PIDs.

The main focus of the case study is persistent identifiers for research instruments and facilities (PIDINSTs from now on), but the analysis is actually aimed to cover any emerging PID infrastructure and thus has links to other PID areas like persistent identifiers for conferences (ConfIDs) and -to a certain extent -to PIDs addressed in other case studies such as IGSNs for samples and ROR IDs for organisational identifiers.

With the Research Organization Registry (ROR) already up and running, the latter area of OrgIDs is clearly more advanced at the time of writing than this just-emerging PIDINST layer, but there are common challenges for both domains to come to fruition. This case study builds on the one devoted to OrgIDs.

One key aspect in this regard is the fact that the use cases for OrgIDs were identified long ago in the report "Review of selected organisational IDs and development of use cases for the Jisc CASRAI-UK Organisational Identifiers Working Group" released in 2015 1 . Since this report was released, the aim has been to identify the most suitable approach to creating an infrastructure and a governance structure that would be able to support at least some of such use cases. On the other hand, the use case analysis for PIDINSTs has not yet been made, and this is something that this case study will try to address.

Same as in the case of OrgIDs and other emerging PIDs, the largest risk perceived at the moment is that of fragmentation and subsequent lack of uptake.

This case study will hence focus on identifying the various existing initiatives exploring mechanisms to implement PIDINSTs and the multiple stakeholders that are simultaneously looking into this area with little coordination across them. The emphasis will then be on the need to figure out coordination and common awareness-raising mechanisms for the community to be able to advance together in this area.

i.

The initial composition of the RDA PIDINST WG is available on pp. 12-13 in reference [START_REF]Case Statement[END_REF] below. It includes numerous representatives for data centres, supercomputing centres and other large-scale research facilities but shows little presence of research funders, research-performing organisations or the wider scholarly communications community.

The case for PIDINSTs

Multiple stakeholders are working on the technical and community-management requirements around PIDINSTs -researchers and funders being the most prominent ones -but the case for PIDINSTs hasn't been clearly outlined yet. This is perceived as a significant risk to the uptake of this emerging PID, since a lack of awareness on the objectives the initiative aims to achieve would obviously impact the willingness of other stakeholders -institutions and publishers -to become involved in it.

A basic analysis on the relevant use cases for PIDINSTs is attempted below, but these suggestions should ideally be validated by a working group that could also serve as a much-needed coordination forum across stakeholders. First it is worth pointing out that PIDINSTs represent a much more ambitious and complex endeavour than author and organisational IDs. Some reasons for this complexity are listed below: ` It may well be possible to implement author ID, OrgID and research output ID layers without a direct intervention of researchers, but this is just not feasible for PIDINSTs. Even if these assets may be paid and operated by research funders and institutions, researchers are the truly knowledgeable stakeholder with regard to research instruments and facilities. It's no coincidence that the PIDINST WG2 running within the Research Data Alliance (RDA) is purely researcher-driven with little if any involvement from research funders and institutions i .

`Research instruments and facilities tend to be jointly addressed in the discussion on PIDINSTs, but these are two clearly different areas. While it may be feasible to figure out a unified approach to their persistent identification, early analyses show that facilities are much easier to address than research instruments.

`Perhaps the main reason why research instruments are much harder to address than facilities is summarised in these two overlapping questions: "What is an instrument?" and "What instruments are relevant enough to be assigned a PID and which ones are not?"

`One of the most evident differences between facilities and instruments is the variability of the landscape a PIDINST initiative would be trying to uniquely and persistently identify. While research facilities are fairly stable pieces of research infrastructure, new instruments are being purchased

As mentioned in the introduction above, there are plenty of concurrent efforts going on to explore the best way to produce a PIDINST layer, but -as opposite to how the Jisc-CASRAI UK WG on OrgIDs addressed the perceived needs in this specific domain -no clear cross-stakeholder framework has been provided to date on the use cases that PIDINSTs would need to address.

and set in operation every year that would need to be identified anew. Many others are decommissioned, and this should also be reflected in a PIDINST record.

The level of curation and maintenance that PIDINST records for research instruments would require is hence much more significant than in the case of facilities.

`Same as in the case of OrgIDs, it is not clearly defined who should own the PIDINST records for curation and management purposes. Institutions may be the best-placed stakeholders for instruments, while funders could naturally be the default choice for taking care of facilities, but the workflows for record curation are simply not in place at this stage.

`The case for the usefulness of PIDINSTs to the wider scholarly communications community is at the moment much weaker than any of the other PIDs under discussion. There is usually a clear-cut main goal for each PID layer -this is unique identification and subsequent disambiguation of researchers for author IDs and unique identification of affiliation for OrgIDs -but in the case of PIDINSTs this is far less clear right now.

`Furthermore, the research information workflows where identifiers for research instruments and facilities would clearly be a very useful asset to have are remarkably far from the usual scholarly communications stakeholders, meaning research libraries. This will be elaborated on in the section on use cases below, but at the moment the activity for collecting information on institutional research facilities and instruments regularly falls within the remit of research offices rather than with libraries, while discussions on metadata sets tend to involve the latter. This poses an additional internal coordination challenge that will need to be addressed.

The RDA PIDINST WG published a "case statement" in Dec 2017 making the case for a "community-driven solution for globally unique and unambiguous identification of instruments instances that are operational in the sciences" More recently, the Department of Economy, Science and Innovation (EWI) of the Flemish Government has started using their Flanders Research Information Space (FRIS) system to explore the feasibility of collecting information on research equipment and facilities held at Flemish universities and beyond by proposing a common metadata set approach for their description, see a slide from a recent presentation above.

It's worth noting that although the first metadata element in this set is an identifier, there is no mention to any international, unique, persistent ID (which of course does not exist right now). These two cases do in fact highlight the risk of landscape fragmentation via national-level initiatives. On the other hand, as seen in the case study for the Dutch DAI, national-level initiatives may be a good way to start progressing while a well-established international effort is able to consolidate.

Also worth bearing in mind is the fact that these use cases for PIDINSTs contain no specific "publisher use case". This means a strong difference with the cases for author IDs and OrgIDs explored in other case studies in this series (and of course for the original Crossref DOIs for publications). In both such cases, the publisher use case was the clearest driver for the emergence of an international layer of PIDs -be it for author disambiguation in publications, or for the correct identification of their affiliations. It could be argued that it is publishers, grouped under the Crossref banner, who have pushed the hardest for the implementation of a PID layer thus far.

Publishers are also mentioned in the use case analysis this PIDINST WG has conducted, but just to state that the references to research instruments and facilities included in submitted manuscripts could also be processed by publishers based on the widespread availability of a PIDINST layer. A further element could in fact be included in manuscript submission systems to allow these references to be provided by researchers, but this is not a particularly solid driver to promote publisher involvement in such an initiative unless there was a clear requirement for this from a critical mass of research funders. DataCite is already involved in this effort around the implementation of PIDINSTs 7 and could perhaps provide some guarantees as an alternative community-driven stakeholder, especially in view of the relevance of the case for asserting dataset provenance within DataCite DOIs for datasets. It remains to be seen though whether an effort to produce an international PID layer can succeed without a direct involvement from publishers.

While publishers are not too much in the picture, a specific, somewhat unique stakeholder is repeatedly mentioned in the RDA PIDINST WG documentationthe institutional instrument database provider.

Instrument manufacturers, also hinted to be a potentially important player for equipment identification purposes, Slide from presentation "Research infrastructures: metadata model & data capturing in FRIS" 6 are another case-specific stakeholder the PIDINST WG work suggests to work with. These stakeholders have no equivalent in the landscape analysis for other emerging PIDs. This illustrates the significantly higher levels of complexity in the area of PIDINSTs. The institutional instrument database providers in particular are public or private initiatives working directly with universities to allow them to provide a snapshot of their available research instruments and facilities. They do not just serve the discoverability and enhanced visibility case study for institutions, but they frequently also include a booking system for requesting research time at the facilities or instruments. The Kit-Catalogue project led by Loughborough University in the UK 8 and later adopted by other HEIs in the country is a good example of such an initiative, but there are many others ii, 9 .

Another case-specific aspect worth pointing out in the PIDINST landscape is the key role played by institutional technicians in the operation and description of research instruments and facilities. These technicians are often shown as responsible for a specific piece of institutional equipment in the available metadata for these. In fact, it's these professional profiles -often overlooked for the purpose of assessing the institutional research output but increasingly brought into the picture by recent studies and reports 10 -who provide the descriptions of institutional research facilities that are being published in discipline-specific outlets like the Journal of large-scale research facilities (JLSRF) 11 . The journal homepage states that:

The Journal of large-scale research facilities (JLSRF) publishes articles describing large-scale scientific equipment. This covers large-scale equipment from all scientific disciplines intended to be used by scientists who are not affiliated to the institution operating the facilities (dedicated user operation). The articles in JLSRF provide scientists with a simple means to reference large-scale facilities in their publications.

ii.

Interestingly, the collection of institutional instrument database provider systems in [START_REF] Duca | Five research equipment booking systems reviewed[END_REF] makes emphasis on the case for booking instrument/facility time at the appropriate equipment as a mechanism to promote equipment sharing and not so much on the (perhaps implicit) objective of increasing discoverability and visibility for the institutional equipment.

The case statement issued by the RDA PIDINST WG cited in the bibliography suggests in fact that the DOIs issued by this journal for its published articles could be used as a sort of persistent identifier for referencing the facilities described. This approach is not fully aligned with the way persistent identifiers typically point at a metadata set describing the object (see an example for an OrgID below) and reveal the fragmented early-stage, strongly researcher-led, bottom-up attempts to identify research facilities.

Some issues identified in the analysis of the early-stage work carried out so far in the area of PIDINSTs are listed below. Some of these will be analysed in more detail in the section devoted to risks and trust at the end of this case study.

`Some types of research facilities and equipment and some countries are overrepresented in the RDA PIDINST WG, while other relevant stakeholders like research funders and universities are largely absent from the discussions. This leads to a somewhat skewed analysis of the use cases and the roadmap for PIDINST implementation.

`Partially as a result of the absence of universities in the discussions, low-hanging-fruit cases for the purpose of persistent identification of (institutional) research equipment and facilities are barely addressed in the WG reports `Research articles summarising the results of the work carried out by this working group have only recently been published 12,13 and haven't yet had the time to influence the developments in the area even if the second reference already includes a suggested metadata schema for describing instruments and facilities.

Technical approaches to PIDINST implementation

The current PIDINST landscape shows a range of approaches towards persistent identification of research instruments and facilities, including DataCite DOIs, OrgIDs and a myriad of internal identifiers within instrument registries and databases mostly kept and maintained by research-performing organisations but also increasingly by research funders in different countries. This fragmented landscape is characteristic of an emerging PID area and doesn't represent a significant issue for the eventual implementation of an international PIDINST layer once the appropriate procedures and workflows are defined, but it stresses the need to find suitable coordination mechanisms across the various stakeholders involved in the endeavour.

The fragmented PIDINST landscape described so far resembles somewhat the attempt to implement a specific, national level author ID in the Netherlands before the arrival of international initiatives in the area like ISNI and ORCID. The case study devoted to the Dutch DAI within this series shows that the replacement and superseding of this early-stage identifier by a mix of ISNIs and ORCIDs posed no major issues in the evolution of the PID landscape in the country.

This section will analyse and provide examples for the various identification mechanisms that are currently being used to generate PIDINSTs. Some suggestions will also be made to more effectively achieve a more widespread -perhaps in terms of reaching a critical mass -PIDINST implementation.

DOIs are the most straightforward choice for persistent identification of research instruments and facilities. They not only provide a well-established, well-functioning infrastructure and governance layer, but DOIs minted by DataCite are also fully aligned with the main case for PIDINSTs, namely tracing the provenance of research datasets also identified by means of DataCite DOIs. But as we have seen for the ITER-Cadarache example above, the choice for DOIs is far from being the only approach, especially for large-scale research facilities operating as standalone research equipment outside institutions.

As a non-profit, community-driven member organisation, DataCite is playing the PID infrastructure provider role for these early-stage PIDINST implementations The DOI-based persistent ID layer for a research instrument mentioned as an example in the DataCite blog post at [START_REF] Buys | PIDs for instruments: a way forward[END_REF] is for one specific seismic network -the Shumagin-East Aleutian Network in the Alaska Peninsula, https://doi.org/10.7914/SN/SH -within the International Federation of Digital Seismograph Networks (FDSN), see figure below. The landing page for this DOI shows a loosely structured metadata collection including among others a description of the network, its geolocation, a suggested citation to be added to manuscripts, the academic institution in charge of its operation and a list of the stations that make up the network. Two elements highlighted in the figure below are the DOI itself and a link that allows the operator of the network to update the information on the page where appropriate.

While this is quite a complex instance of a research instrument, the feature enabled by the FDSN to allow the operating institution to curate and maintain the record for the facility is a valuable example for a curation workflow that could be applicable to other emerging PIDs too.

Although the process is not complete and plenty of networks do not yet have a DOI, the FDSN has created further DOIs for other seismic networks within the federation such as the ones listed below. Note that the authors' names on the citations are for the operators of the specific network, following the practice established by the JLSRF at [START_REF]Journal of large-scale research facilities[END_REF]. This might suggest diverse options for a persistent identification of research instruments and facilities, especially bearing in mind the approach described in the case study devoted to OrgIDs whereby multiple-level ROR IDs could potentially be used to assign PIDs to instruments held within these large-scale research facilities.

`

The role of Current Research Information Systems (CRIS) for the implementation of PIDs

As shown in the Directory of Research Information Systems (DRIS) maintained by the non-profit member association euroCRIS 18 , there are over a thousand operational Current Research Information Systems worldwide devoted to capturing all aspects of the research activity carried out at research-performing organisations, mostly universities. These systems are widespread in most European countries and particularly in the six Knowledge Exchange member countries.

CRIS systems, also known as Research Information Management Systems (RIMS), often operate on the basis of the CERIF standard (for Common European Research Information Format) for metadata description purposes 19 . CERIF is composed of a network of interconnected entities, with persons (researchers), organisations and projects at its core. Research equipment is a secondary-level entity in the CERIF data model, and it is linked to the core ones and to various other entities. This is essentially the same concept as the research graphs for describing research and specifically the same as PID graphs using persistent identifiers to establish such interconnections across entities. The recently released [DataCite] Metadata Schema for the Persistent Identification of Instruments 13 is in fact well-aligned with the structure of the CERIF cfEquipment entity.

CRIS conferences and membership meetings held by euroCRIS in the past have seen contributions in the area of research information management via persistent identifiers and specifically on collection of information on research instruments and facilities, see for instance [START_REF] Cox | equipment.data -Delivering a data autodiscovery infrastructure[END_REF] and [START_REF] Tatum | Principles and Pragmatics of "as open as possible": persistent identifiers as the interface between research information commons and closed systems[END_REF]. Because these events are usually attended by CRIS managers both for institutional and national/ regional CRIS, they could represent a good opportunity to make the case for the implementation of emerging PIDs such as PIDINSTs, to discuss the associated workflows and to exchange good practices in the area.

Moreover, institutional CRIS systems very often contain records for institutional research equipment, see an example in the figure below for the list of instruments and facilities available at the institutional CRIS at the University of Strathclyde in Glasgow, UK. The CERIF data model underpinning these institutional CRISs already allows publications and datasets to be linked to instruments and facilities available in the institutional database -even if the provenance equipment does not yet have persistent identifiers associated with it. The UUID structure underlying these CERIF entities already allows these links to be set.

As a result of this, PIDINSTs for the available equipment could quickly be made available by using the DataCite services for DOI minting that member institutions are already using for minting DOIs for datasets. However, this would be a largely pointless exercise if the new PIDINST layer were not underpinned by a default use case for using such persistent identifiers to accurately reference instruments and facilities in -for instancemanuscripts submitted for publication and their underpinning datasets.

This need for services to be developed on top of an eventual PIDINSTs layer suggests research funders might be the key driver for their adoption. By requiring this additional feature -same as ORCIDs were required in past iterations of the gradual emergence of a PID landscape -funders would be able to promote the citation of instruments and facilities by researchers in their research publications and datasets. Funders would also be able to obtain usage metrics for the research instruments and facilities they are funding at institutions. Publishers could also explore the required mechanisms to include references to PIDINSTs in manuscripts and journal articles. This means a significant cultural change, one that would require researchers to apply a systematic approach to citing PIDINSTs, but this could potentially be supported by the gradual inclusion of the maintenance and operation of institutional research instruments and facilities within evaluation frameworks for universities like the Knowledge Exchange Framework (KEF) in the UK.

Issues around risks and trust regarding the implementation of PIDINSTs

Serious risks associated with the implementation of PIDs for research instruments and facilities are perceived to be the fragmentation and lack of coordination across initiatives working in the area, which could in turn result in an uneven uptake. This is a common trait to all emerging PIDs, but in the case of PIDINSTs, the landscape fragmentation does not only involve significant methodological variations across research disciplines, but also the fact that whole stakeholders within the scholarly communications community are currently unaware of the recent progress in this domain. In order for this sort of PIDs to consolidatemeaning not just PIDINSTs but also the additional 'technical' PIDsputting them firmly on the radar of actors like research funders and institutions is seen as a must.

The very early-stage implementation of PIDINSTs at the time of writing raises a good number of potential risks and trust-related issues. Some of these -such as the risk of fragmentation and lack of uptake by the wider community -are common to any emerging PID. More specifically, PIDINSTs are the first example in this series of case studies for what we call a 'technical PID', meaning that its implementation has so far mainly being driven by researchers and research facility managers with little involvement from other stakeholders like institutions or research libraries (ISGNs for geo samples are another example for 'technical PIDs' addressed in this series). This is in contrast to the more 'admin-oriented PIDs' whose use cases much more clearly serve the objectives of the wider scholarly communications community. These are not clear-cut categories, but given that the risks are common for PIDs included in any of such groups, it's worth looking into this classification in a bit more detail.

'Technical' vs admin-oriented PIDs

There is a dichotomy within the current and emerging PID landscape between what we could call "technical" and "admin-oriented" identifiers. Technical PIDs are promoted as bottom-up workflows by researchers who perceive the need for the persistent identification of objects they're regularly working with, be it geo samples (IGSN), research equipment and facilities (PIDINSTs) or clinical trials (ISRCTNs). Admin-oriented PIDs on the other hand (such as -among others -ORCIDs, OrgIDs and the emerging ConfIDs) are implemented in a more (if not completely) top-down fashion by a range of stakeholders that do not include researchers -typically institutions, publishers and research funders -in order to introduce some much-needed standardisation in the scholarly communications landscape for research information management purposes.

The workflows for the implementation of these adminoriented PIDs tend to be much more coordinated across countries than the usually rather fragmented attempts to promote technical PIDs. It is this international coordination what allows researchers to eventually use and benefit from the admin-oriented PIDs too, but at the moment there's no clear roadmap for the adoption of the technical PIDs by the stakeholders in the scholarly communications domain. The community management role that actors like DataCite and Crossref may play in this regard is seen as critical. However, there's still some way to go in terms of making the case for the usefulness of technical PIDs so that stakeholders sitting on the admin-oriented PID side of the landscape may be persuaded to promote and support these other technical PIDs as well. Without such coordination between both sides of the landscape, the risk of fragmentation and lack of uptake will remain paramount and may well prevent the emergence of a widespread PID layer for technical PIDs due to the sheer lack of awareness.

Community engagement

The very successful international implementation of ORCID as a default PID for researchers has made it a certain blueprint for other PID implementation initiatives to follow. This would mean an integrated approach to persistent identification of a specific range of objects by the many different actors that make up the scholarly communications community. However, different PIDs could also follow different strategies for becoming widely adopted.

This said, it is clear at this point that there is a high risk of fragmentation and lack of uptake in the PIDINST domain. There are many parallel initiatives underway which overlap with each other with little awareness of the progress made by other stakeholders, and there's a clear need for mechanisms to bridge the gap between research communities and stakeholders. This initiative by the Knowledge Exchange to analyse the current PID landscape and explore its risks and trust-related issues could be of much help in this regard, but further efforts are clearly required to achieve a widespread adoption and subsequent use of PIDINSTs.

Part of this pending work could be conducted by organisations like DataCite, the EOSC or euroCRIS by organising events where the current status of a given PID domain could be explored with presentations from the pioneering initiatives iii . The purpose of these events would be to raise awareness and bridge the gap across communities, fostering a wide-ranging discussion where actors like research funders and institutions could consider the potential benefits of joining the PIDINST bandwagon.

It's worth pointing out in this regard that several countries have embryonic-stage databases of research infrastructure for admin purposes that could perhaps be upgraded to Things like datasets and instruments -that's information we [funders] are not necessarily capturing as part of the digital application. Details that may be produced as part of one of the outputs and where these are being stored. That information may not necessarily come back to us, it may be

iii. An open webinar on persistent identification of instruments was held on May 19th, 2022 by the RDA PIDINST WG at https://www.rd-alliance.org/PID-instruments-May2022_webinar. This is very much along the lines of the dissemination initiatives suggested here.

deposited with another organisation and then we [would] have to connect to it. So I think there are even greater technical barriers to implementing those smaller PIDs. We are not going to be anywhere close to becoming the central depository for all PIDs, at least currently. Of course, we may never be that. There's certainly no roadmap for that. And so it may be a provider holds all the data sets, a different provider holds all the equipment and instruments, a provider holds the policy outputs, for example, if that becomes one additional area. And [as funders] we have to find a way to connect that information ourselves, but not everyone's in the position to be able to do that.

PIDINST-supported assets and turned into the basis for well-accepted practices for instrument citation in datasets and publications.

A specific recommendation in this regard would be for a research funder in a Knowledge Exchange member country where early-stage PIDINST adoption initiatives are relatively consolidated (such as the United Kingdom or Germany) to consider a funding call for a project involving not just the pioneering, discipline-specific frontrunner initiatives but also other actors such as universities, institutional instrument database providers and even instrument manufacturers. The main goal of such a project would be to create new crossstakeholder collaborative networks and to explore the workflows for the extension of early-stage PIDINST adoption practices onto more mainstream actors such as universities. It would also be for this kind of diverse project consortium to deliver a comprehensive description of the various use cases for PIDINSTs -a case currently somewhat biased due to the lack of direct input from Higher Education Institutions and research funders.

Technical divergence

Due to the siloed nature of these early-stage, discipline-specific attempts at adopting PIDINSTs, a risk of technical divergence is presently looming over the whole domain. Several examples have been provided above on how different initiatives are using different PID standards such as DataCite DOIs, ROR IDs and DOIs for articles on research facilities published in specific journals as the basis for the persistent identification of research instruments and facilities. Part of the issue is due to the perhaps overambitious objective of addressing facilities and instruments as a single area, where these two can be seen as different domains to be independently addressed. This issue has also surfaced in the case study on OrgIDs, where competing standards such as ROR and Ringgold are due to coexist, and may not be a critical one as long as there's an overarching strategy to bridge the gaps across technical standards, but PIDINSTs are currently at a much earlier stage of implementation and this risk is seen to be more acute than in the case of OrgIDs. This is in fact a wider issue for emerging PIDs that will also affect grantIDs, an area where early-stage approaches are mainly being supported by Crossref while RAIDs are simultaneously coming into the picture too.

We've also had various conversations around instruments and we do support that, at a very basic level at this stage. But yeah, also something to consider. So I would say, my advice, because I can't give you an exhaustive list, would be to assess the use cases that are needed, or what use cases you are trying to address. And then do an analysis of what is available there and then overlay that with the trust or maturity of those services and communities that would offer those. And that's what we are trying to do in partnering and working with different organisations, to elevate the trust and maturity of these services and scale more efficiently.

The [effort around] instruments is an interesting one, because something that we [publishers] very much focus on is scalability and consistency of approaches. Now, instrumentation is very, very different from field to field and the tracking of that, I mean, we look at astronomy, they've done it incredibly well, for 20-odd year. That gives them a great consistency of data and understanding. But then if we flip over into something like some of the chemical sciences, it becomes very variable within sub-disciplines of chemistry. And so, if we could get to a place where there was some kind of identifier that worked across disciplines, it would be very beneficial, especially for abstracting, understanding of impact, identifying the ongoing types of research and trends and so on. I think the likelihood of that happening is much lower than in something like institutions or research objects or so on, because the variability is so high. Yeah, so I think it would be beneficial, but it's more unlikely.

In a way these risks of technical divergence are part of the issues around community engagement mentioned in the previous section and could probably be addressed as part of a more general awareness-raising exercise. It is important in any case to bear them in mind in order to avoid fragmentation.

Overload of PID infrastructure providers

Emerging PID domains like OrgIDs, PIDINSTs, IGSNs, grantIDs and ConfIDs are likely to be simultaneously developed and matured. The fact that the underpinning PID infrastructure and the community coordination for all these initiatives sit with a single actor in the community -namely DataCite -is also seen as a significant risk.

This would be about an overload for the servicesespecially in the area of community engagement and management -to be provided by a single organisation. This risk could result in a slowing down of the progress in the implementation of these emerging PIDs and the inability to counter the natural fragmentation that tends to arise from early-stage efforts. As mentioned above, the participation of other stakeholders like the EOSC in the efforts to define a standardised, coordinated roadmap towards a widely implemented PID layer may well counter this risk of overload. Projects conducted under the EOSC umbrella like FAIRsFAIR and the recently started FAIR-IMPACT and FAIRCORE4EOSC ("Developing a set of EOSC-Core components to enable a FAIR EOSC ecosystem", 2022-2025) led by CSC in Finland with DataCite among its partners do in fact mean an opportunity to jointly address this challenge of pushing for the simultaneous implementation and consolidation of many different emerging PIDs.

  7 . This is partly a result of the collaboration networks established in the course of the EU-funded FREYA-777523 project ("Connected Open Identifiers for Discovery, Access and Use of Research Resources", 2017-2020). The section in the FREYA project website devoted to "prototypes of new PID services" 14 includes two subsections on PIDs for scientific instruments and PIDs for research facilities. Project partners PANGAEA and the UKRI Science and Technology Facilities Council (STFC) are referenced as frontrunners in the implementation of PIDs in each of both areas.

  ) at the time of writing 4 . The main use cases for such initiatives are to improve visibility and discoverability of such assets (item 5 in the bullet point list above) and above all to foster the sharing of such equipment hosted at specific institutions with external stakeholders. A series of posts highlighting specific pieces of research infrastructure at various UK universities was published in Jisc's "Sharing Research Equipment" blog 5 alongside the equipment.data database.

	Some funders in Knowledge Exchange-member	These use cases, which are not associated to specific
	countries are in fact already maintaining national-level	stakeholders, include:
	databases of instruments available across institutions in
	the country -see for instance the database equipment	`Metrics that quantify the use of instruments and the
	data promoted by the Engineering and Physical	rationale for future funding
	Sciences Research Council (EPSRC) and Jisc in the UK
	with 17,538 items (instruments) from 51 UK	`Link data to the instruments that generated them
	organisations (universities	(provenance), improving the interpretation and
		validity of data
		`Aid equipment logistics and mission planning
		`Facilitate interoperability and open data sharing,
		especially in advancing technologies that foster
		sharing of instruments
		`Improve the discoverability and visibility of
		instruments and their data, published on the web
		The two strongest use cases on this list are the two first
		ones, which mainly serve the needs of research funders
		and institutions. Both the wish to collect usage data
		(including for collaborations between Academia and
		Industry) for often very expensive research facilities and
		instruments and the Open Science-aligned attempts to
		link research datasets to their instruments of provenance
		could be considered to fall within the remit of funders
		and institutions.

3

. A series of use cases for these PIDs for research instruments and facilities was sketched that remains the closest attempt available to date to answer the question "Why do we need PIDINSTs?"

  Erin Pettit, Ted Scambos, & Martin Truffer (2015). RAPID: Observing the Disintegration of the Scar Inlet Ice Shelf. International Federation of Digital Seismograph Networks.

	Building on this collaborative network, this consortium	Witte, Hannelore (2018). Processed 2 minutes-increasing number of research facilities are also getting
	of facilities in the high-energy physics discipline are now	averaged continuous VM-ADCP (vessel-mounted top-level OrgIDs in a parallel process. The ROR ID for
	running two H2020-funded projects under the umbrella	Acoustic Doppler Current Profiler) profiles during ITER-Cadarache has been shown earlier in this case
	of the European Open Science Cloud (EOSC), namely	Polarstern cruise PS82. Alfred Wegener Institute, study, and other facilities with associated ROR IDs include:
	PaNOSC-823852 ("Photon and Neutron Open Science	Helmholtz Centre for Polar and Marine Research,
	Cloud", 2018-2022) and ExPaNDS-857641 ("EOSC	Bremerhaven, PANGAEA, `the Diamond Light Source at
	Photon and Neutron Data Services", 2019-2023).	https://doi.org/10.1594/PANGAEA.887545 https://ror.org/05etxs293 or the SKA Observatory,
		https://ror.org/01pwgd096, in the United Kingdom,
		The metadata set for this persistently identified
		PANGAEA dataset includes links to the Polarstern `the German Electron Synchrotron (DESY) in
		research vessel described at the JLSRF, to the specific Germany, https://ror.org/01js2sh04,
		campaign (PS82) via a TIB-hosted DOI-based ID and
		also to the specific research instrument that was used `the European Spallation Source in Sweden,
		on the vessel, Acoustic Doppler Current Profiling https://ror.org/01wv9cn34,
		(ADCP), TRDI Ocean Surveyor, 153.6 kHz,
	Bed Access,
	Monitoring and Ice Sheet History (BEAMISH,
	2016-2019) [Data set]. International Federation of
	Digital Seismograph Networks.
	https://doi.org/10.7914/SN/9B_2016
	As mentioned above, PANGAEA was a FREYA project
	partner in the domain of PIDs for research instruments
	and facilities and is thus a frontrunner in the
	implementation of PIDINSTs, see for instance the DOI
	for the Polarstern research vessel stemming from an
	article devoted to this research facility published in the
	JLSRF:
		Another very relevant research discipline for the
	Alfred-Wegener-Institut Helmholtz-Zentrum für Polar-	implementation of PIDINSTs is that of high-energy
	und Meeresforschung (2017). Polar Research and	physics, and specifically the trans-European community
	Supply Vessel POLARSTERN Operated by the	of national Neutron and Photon facilities. The well-
	Alfred-Wegener-Institute. Journal of large-scale	established collaborative network across these facilities
	research facilities, 3, A119.	and institutions formally started with the PaNdata
	http://dx.doi.org/10.17815/jlsrf-3-163	ODI-283556 FP7 project in 2011 ("Photon and Neutron
		Data -Open Data Infrastructure", 2011-2014). The main
	Moreover, datasets hosted by PANGAEA are already using	project goal was to establish a data infrastructure,
	the reference to both the research facility and the research	federated among the European Neutron and Photon
	instrument from which the data was obtained, see	facilities, to enable the scientific communities access,
		analysis and sharing of scientific data in a collaborative
		research environment.

https://doi.org/10.7914/SN/1T_2015 `Heit, B., Yuan, X., Almendros, J., Abella, R., Carmona, E., Aguí, F., & Carrión, P. (2020). BRAVOSEIS Onshore Seismic Array. GFZ Data Services. https://doi.org/10.14470/0Z7563857972 `Alex Brisbourne, & Andy Smith (2016). https://doi.pangaea.de/10013/epic.47834.d001. This link gives access to a 2-page instrument description by the manufacturer Teledyne RD Instruments in California. It is no coincidence that -same as for the IGSN emerging PID addressed in another case study in this series -both examples for early-stage implementation of PIDINSTs belong to the field of Earth Sciences. It is this area, with its large data banks

15

, where the case for linking datasets to their provenance is most acutely felt at the time. At the same time, these are fairly sophisticated examples of persistent identification for instruments and facilities, and it's worth wondering how these early-stage practices may be taken over by more down-to-earth instances like universities. A suggestion in this regard is made in the following section. As shown on the figure below, taken from a Nov 2020 PaNOSC/ExPaNDS project presentation 16 , persistent identification of research facilities and instruments is very much on the radar of this research community under the concept of "FAIR research infrastructures". The PaNOSC/ExPaNDS network has in fact recently held specific workshops on the topic of PIDs for research facilities 17 . Same as in the case for the early-stage PIDINST implementation in the Earth Sciences above, this 'niche' HEP research network is detached from the mainstream scholarly communications community at universities. The way these efforts are being conducted suggests however that the EOSC and specifically its PID Policy and Implementation Task Force could play a significant role in the promotion of PIDINSTs via the various EOSC science demonstrators in multiple disciplines that were carried out under the EOSCpilot project.

The examples for PIDINSTs explored so far are all based on DOIs as a persistent identifier type. However, an `the Barcelona Supercomputing Center (BSC) at https://ror.org/05sd8tv96 or the Large Canary Islands Telescope, https://ror.org/03efc2j36 in Spain.

The case for PIDINSTs

Issues around risks and trust regarding the implementation of PIDINSTs

Authorship