TCNQ Grown on Cu - American Chemical Society

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TCNQ Grown on Cu (001): Its Atomic and Electronic Structure Determination M. J. Capitán,*,† C. Navío,‡ J. I. Beltrán,§ R. Otero,‡,∥,⊥,# and J. Á lvarez*,∥,⊥,# †

Instituto de Estructura de la Materia-CSIC, 28006 Madrid, Spain Instituto de Estudios Avanzados en Nanociencia-IMDEA. 28049 Madrid, Spain § IMDEA Materials Institute, c/Eric Kandel 2, 28906 Getafe, Spain ∥ Departmento de Física de la Materia Condensada, Facultad de Ciencias-UAM. 28049 Madrid, Spain ⊥ Instituto Nicolás Cabrera, Facultad de Ciencias-UAM. 28049 Madrid, Spain # IFIMAC-Condensed Matter Physics Center Facultad de Ciencias-UAM. 28049 Madrid, Spain ‡

ABSTRACT: In this paper we have resolved by means of “in situ” X-ray diffraction studies the atomic structure of the TCNQ films from 0.4 to 17 ML thickness grown on Cu(001) substrate. The film grown at low temperature, is a well crystallized and well oriented single phase. The TCNQ film has a (020) orientation. The electronic properties of the film have been studied by means of XPS and UPS spectroscopies. The measured electronic density has been compared to the theoretical ab-inito calculations. In this paper we have studied the structural and electronic properties of relatively thick films of TCNQ on Cu(100). Our results demonstrate that the strong charge transfer between the metallic substrate and the first layer of TCNQ molecules have important consequences for the film electronic and structural properties of films even as thick as 17 ML. From the structural point of view, the films grow with a well-defined (020) orientation that matches the modified molecular structure of the first layer. This first monolayer structure was previously explained in literature in terms of adsorbate-induced reconstruction of the Cu surface. From the electronic point of view, charge transfer leads to band bending and determines that barrier for electron injection across the metal/organic interface.

1. INTRODUCTION

identifying their electronic and charge state and, eventually, their magnetic behavior. For the application of these materials in practical electronic devices, it is necessary to fabricate these materials into thin films and to clarify the electrical properties of the resulting film and their dependence on the film structure. To date, the possible industrial applications of these films have been proposed in coverages ranging from nanometers to micrometers thickness. The configuration and registry of TCNQ molecules in such thick films on electrode surface is a fundamental issue in both the fundamental research and industrial application. However, there is still some controversy in the literature about the thin film structure and its dependence on the preparation method and furthermore, the influence that the structure has on its properties. Works present in the literature show that the TCNQ film crystallinity and texture depends strongly on the used growth method, the substrate and even on the experimental growth conditions. Thus, Kojima et al.7 grew a crystalline TCNQ-film by evaporation under vacuum on glass substrate when it is

During the last few decades, tetracyanoquinodimethane (TCNQ) and its family of small polynitriles have been of much interest due to their ability to form complexes with both organic and metallic donors that exhibit wide varieties of optical, magnetic and electric properties, resulting in an increasing application in the development of molecular-scale devices.1−12 A large number of these TCNQ studies are aimed at the understanding of the large strength of TCNQ as an electron acceptor and its ability to form stable charge-transfer complexes with both organic and metallic donors and its role in the optical and electrical properties of the compound. Furthermore, the acceptor properties of TCNQ arise from the strong electrophilic character of its two cyano terminations. Thus, the interaction of the cyano groups with d atomic orbitals leads to the extraction of one electron from the metal, which is accordingly placed into a molecular orbital, forming a radical anion molecule.13 The unpaired electron added to TCNQ through a metal−organic bond bestows magnetism to the molecule, which gives also promising applications in the synthesis of organic magnetic materials.14−20 It is then of fundamental interest to explore the properties of TCNQ and its related components on the surface of a metal, aiming at © 2016 American Chemical Society

Received: September 6, 2016 Revised: November 15, 2016 Published: November 15, 2016 26889

DOI: 10.1021/acs.jpcc.6b08999 J. Phys. Chem. C 2016, 120, 26889−26898

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The Journal of Physical Chemistry C grown at 280 °C, but it grew amorphous at a lower temperature. The crystalline film obtained by them is oriented along (022) direction indicating that the TCNQ molecules lay flat to the substrate surface. In the same direction, El-Nahas et al.21 get an as-deposited policrystalline film on glass substrate, but when annealing, the (022) orientation also becomes the preferred one. Tseng et al.,22 show that the growth of TCNQ is sensitive to the nature of the substrate. Thus, on weakly interacting substrate (glass) they get a randomly oriented microsized island with three main orientations: (022), (021), and (020). However, on substrate that has stronger interactions with the TCNQ molecules (glass precovered with 3,4,9,10perylenetetracarboxylic dianhydride (PTCDA)), they have uniform rectangular-shaped microsized islands with only (020) molecular orientation. Previous studies had demonstrated that the strong interaction between the first layer of TCNQ molecules and the Cu(100) surface leads to a strong reshaping of both sides of the interface, which leads to new substrate-mediated intermolecular interactions. Such interactions allow the growth of well-ordered and large domains of TCNQ islands for coverages below 1 ML.23 Here we show that these first layer structures act as a template to facilitate the growth of crystalline films with thicknesses between 2 and 17 ML, even for lowtemperature deposition, and displaying a well-defined growth orientation. Moreover, the transfer of electrons from the metal substrate to the first layer molecules leads to the bending of the bands close to the interface and, thus, determines the charge injection barrier across the interface. We study the structure and electronic properties of TCNQ thin film grown on Cu(001) surface using a low deposition temperature. Our results demonstrate that TCNQ exhibits a strong tendency to form highly ordered molecular assemblies with a structure and orientation that matches a formed interface layer. This interface is formed due to the strong TCNQ−Cu interactions and it is compatible with the structure given in literature23 for the first TCNQ monolayer grown on Cu(001). The interface layer seems to remain localized at the metal− TCNQ interface and has a special electronic property which comes from the injection of a d-metal electron, close to the Fermi level, to the TCNQ molecules empty states. The electronic properties of the film have been measured by XPS and UPS and they have been compared to the theoretical calculated density of state.

CuTCNQ grown on Cu(100) has been used as reference for the photoemission studies. This sample is used in order to recognize by photoemission whether there is a strong interaction of the TCNQ molecule with Cu and their associated change in core level and density of states (UPS) spectra. The UHV evaporation cell was provided with a large beryllium window (transparent to X-rays) to allow simultaneous X-ray diffraction (XRD) measurements. The TCNQ structure was characterized by means of surface X-rays diffraction at a fixed incidence angle of 2°. These measurements were performed at the W1.1 beamline at Hasylab synchrotron at Desy and at the I811 beamline at MAX-lab IV at Lund, using a wavelength of 1.397 Å. However, in the θ/2θ plots shown in this work we have standardized the experimental diffraction patterns to the Cu Kα wavelength in order to be comparable to the diffraction patterns present in the literature (in Figure 2A). The experimental setup has a six circle goniometer to allow a diffraction geometry with fixed incoming beam angle onto the crystal surface. A sketch of the geometry is shown in Figure 1.

Figure 1. Sketch of the diffraction geometry with respect to the substrate. The parallel (Q∥) and perpendicular (Q⊥) momentum transfer magnitudes are shown with respect to the substrate surface. The kin and kout show the incident and outgoing beam.

The experimental resolution was defined by a pair of slits between the sample and the 0D-scintillator detector placed on a large detector arm (1.2 m of sample−detector distance). Figure 1 describes the different geometrical magnitudes used in the diffraction studies. In order to make a deeper study of our film morphology we have made scans at different fixed outgoing beam angles (γ) with respect to the surface. Thus, in these scans the detector arm is running parallel to the substrate surface. This scattering geometry is widely used in close to the surface X-ray diffraction. Instead of the outgoing angle (γ) the use of other related magnitudes which are more lineal with the lattice distance is most extended; either the perpendicular momentum transfer parameter (Q⊥, being Q⊥ = 2 sin γ/λ) or, ls which is the Q⊥ scaled by the substrate lattice parameter in the perpendicular to the surface direction (ls= Q⊥cCu(001) where s refers to substrate). Thus, ls is an adimensional magnitude which is inversely proportional to the relative size of the film lattice with respect to the substrate lattice in the perpendicular to the surface direction. In our case, the substrate is the Cu (001); thus, ls = 1 corresponds to a lattice equal to the c

2. METHODS 2.I. Experimental Methods. TCNQ (7,7,8,8-tetracyanoquinodimethane) powder from Sigma-Aldrich was used in the experiment. The TCNQ films were deposited on Cu(001) single crystal under ultra high vacuum (UHV) conditions (base pressure 2 × 10−10 mbar) maintaining the substrate at −50 °C by means of nitrogen liquid circuit. We prepared Cu(001) single crystal by “in situ” Ar+ sputtering and flash-annealing cycles under UHV conditions. As a result of this, a sharp diffraction pattern could be observed for the copper substrate, including copper surface diffraction rods. TCNQ was deposited by thermal evaporation from glass crucibles at a pressure of 1 × 10−8 mbar, which corresponds to a crucible temperature of 65 °C. Under these conditions, the growth rate for TCNQ were measured to be 0.2 ML/min (monolayer/min). Samples of different thickness were grown, but here we show the results for three of them, one below the monolayer coverage, (0.4 ML) and two above (1.8 and 17.1 ML). A sample of 19 ML of 26890


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The Journal of Physical Chemistry C

Figure 2. (A) The blue dotted patterns is the measured XRD data at fixed outgoing beam ls = 0.7 for the 17.1 ML thick TCNQ film. The black line is the calculated powder diffraction pattern for the monoclinic TCNQ crystal.28 (B) XRD patterns of this 17.1 ML thick TCNQ film measured as a function of the total momentum transfer (QT = 2π/d) for different experimental perpendicular momentum transfer in units of ls (perpendicular surface momentum relative to the substrate perpendicular parameter). The blue line corresponds to the same pattern that is shown in part A. The blue arrow is the (112) Bragg peak and is used as a reference eye-guide between parts A and B. (C) Sketch of the TCNQ crystal orientation with respect to the substrate (on top) and of the grain shape and orientation of the film mosaic blocks (on bottom) Both model shapes were deduced from their corresponding X-ray diffraction patterns shown in part B.

parameter of the Cu crystal (being cCu(001) = 3.61 Å). On the other hand, the parallel momentum transfer is related to the angle of the outgoing beam within the substrate plane (δ). The total momentum transfer is the modulus of the parallel and perpendicular momentum transfer (Q = |(Q∥,Q⊥)|) and is related to the plane distance and the total diffraction angle (θ) by the Bragg law (Q = 1/d = 2 sin θ/λ). The electronic properties were studied with a hemispherical energy analyzer (SPHERA-U7) and using a monochromatic Al Kα line source (hν = 1486.7 eV) for the X-ray photoelectron spectroscopy (XPS) studies and an ultraviolet He discharge lamp for the valence band measurements (ultraviolet photoelectron spectroscopy, UPS). Both He I (hν= 21.2 eV) and He II (hν = 40.8 eV) lines were used for the UPS measurements. The analyzer pass energy was set to 20 eV for the XPS measurements to have a resolution of 0.6 eV, whereas for the UPS the pass energy was set to 5 eV corresponding to a resolution of 0.1 eV. All the core levels are referred to the Cu 2p3/2 peak of the clean substrate (binding energy of 932.3 eV) and the UPS spectra to the Fermi edge of the clean Cu substrate. 2.II. Theoretical Calculations. Density functional theory (DFT) calculations were performed employing the VASP code24−26 and the PBE functional for the exchange correlation term.27 Pseudopotentials were used for the description of the core levels while the following oribitals were treated quantum mechanically: 3d10 4s1 (Cu), 1s1 (H), 2s2 2p2 (C), and 2s2 2p3 (N). The employed kinetic energy cutoff is 400 eV, and all atomic positions are fully relaxed until the atomic forces were below 0.01 eV/Å. The experimental monoclinic structure of the TCNQ (a = 8.910 Å, b = 7.068 Å, c = 16.403 Å, and β = 98.51°) was used as initial bulk configuration.28 This is also true for the slab calculation of 1 ML TCNQ (010) on Cu (001), where the vacuum distance is around 14 Å, to study the

interaction between the TCNQ and only one Cu surface. The K-point mesh used for the bulk and slab calculations was 3 × 4 × 2 and 3 × 1 × 2 respectively in the Monkhorst−Pack scheme.29 After relaxation, the calculated lattice parameters and β angle for the TCNQ bulk were within 3% and 0.3% of the experimental, respectively.

3. RESULTS AND DISCUSSION 3.I. The TCNQ Thin Film Structure. The structure of the deposited TCNQ film was studied by means of “in-situ” X-ray diffraction (XRD). The sample holder temperature was kept at −50 °C throughout the whole experiment. In Figure 2A, the experimental pattern of the TCNQ film of 17.1 ML thick is compared to the calculated powder diffraction pattern for a monoclinic TCNQ structure given by Long et al.28 According to these authors the TCNQ has C2/c monoclinic lattice with a = 8.906 Å, b = 7.060 Å, c = 16.395 Å, and β = 98.54° cell parameters. Works present in the literature show that the TCNQ film crystallinity and texture depends strongly on the used growth method, the substrate and even on the experimental growth conditions. Depending on the strength of the interaction between TCNQ molecules and the substrate the resulting film can be amorphous, policrystal or highly textured. In order to perform a deeper study of our film morphology, we have made scans at different fixed outgoing beam angles perpendicular to the surface (γ) (or equivalently at different fixed ls). In Figure 2B, we show these measured patterns as function in total momentum transfer units (Q = |(Q∥,Q⊥)| = 1/d) for the different fixed experimental ls (see experimental method section for a more detailed description of these magnitudes). The ls is inversely proportional to the ratio between the film and the substrate lattice parameters perpendicular to the surface direction. It can be observed that the peak intensity changes 26891


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Figure 3. (A) STM image of TCNQ over Cu(001) on a submonolayer thick coverage and proposed 2D lattice deduced from this image. (B) Structure model of the a monolayer slab of TCNQ (020) plane grown on Cu(001) view in a perpendicular to the substrate surface (left panel) and in a traversal view (right panel). (C) Structure model of a monolayer slice of TCNQ (022) grown on Cu(001) view perpendicular to the substrate surface (left panel) and in a traversal view (right panel).

almost the half. Thus, all these intensity changes can be explained by the fact that the TCNQ film has a preferential (020) orientation. Thus, the TCNQ molecules are stacked along its b-axis direction perpendicular to the substrate surface (see sketch in Figure 2C). In the TCNQ (020) orientation, the TCNQ molecules are stacked along the perpendicular to the substrate direction in such a way that they have a relative displacement and shift with respect to the TCNQ ring plane (see Figure 2C). Thus, between two TCNQ molecules belonging to different layers, there is a displacement in the perpendicular to the molecule πplane but also a shift in the parallel direction. Huang et al.30 have shown by theoretical calculation that this packing maximizes the attractive force due to intermolecular overlap (π−π interactions) while keeping the repulsive force due to close interatomic contacts (repulsion between the cyano groups) minimal. The X-ray diffraction peak width is related to the mosaic block size by means of the Sherrer formula. Thus, in the initial TCNQ powder pattern (not shown) it has very narrow Bragg peaks, which indicates that the mosaic block size is very large

with the perpendicular momentum transfer used in each scan (∝ls). There are peaks that decrease their intensity by increasing the momentum and vice versa. Using the Long et al. TCNQ lattice model28 it is possible to index all the TCNQ thin film peaks. Some of these Miller indexes for each Bragg peak are given in Figure 2B. Although some of the Bragg peaks correspond to more than one family of planes (different Miller index) we have pointed out some significant ones. It is clear that the peak intensity evolution with the perpendicular momentum depends on this Miller index of the Bragg peak. Thus, the Bragg peak associated with K = 0 (TCNQ(H0L) family of planes, red color indexes) mainly decrease in intensity with ls and, in opposition to this, the Bragg peak associated with K = 2 (TCNQ(H2L) family of planes, violet color indexes) mainly increase their intensities at the higher ls values. The Bragg peaks related with the K = 1 (TCNQ(H1L) family of planes, black color indexes) show a maximum of the intensity at ls = 0.5 approximately which means that the TCNQ film lattice is twice the substrate lattice in the perpendicular direction. Considering that the substrate parameter in this direction is 3.61 Å and that the bTCNQ = 7.060 Å the ratio between them is 26892


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Figure 4. Comparative of the structure of a monoclinic TCNQ(020) monolayer slab (on the left) to the structure measured by STM in the submonolayer regimen (on the right). The dark red arrow shows the (movement that the 2D lattice suffers in the submonolayer regimen.

(1700 Å). In the TCNQ thin film the peaks are broader. The calculated mosaic block size from the Bragg peak width is 360 Å. Thus, the TCNQ film of 17.1 ML thick growth at −50 °C is a single well crystallized phase with a relatively large mosaic block size. Furthermore, the film is highly textured with the TCNQ (020) lattice direction laying parallel to the substrate Cu (001) surface direction. We can concluded that this film is a very well crystallized, textured and homogeneous. In order to understand the high crystalline quality of our films grown at low temperature, in Figure 3 we compare an slab of the well-established monoclinic TCNQ (020) molecular plane which is the stacking structure already determined (Figure 3B) with the STM images measured on a submonolayer thick TCNQ film (Figure 3A). It is clear that both structures are not the same. The 2D (2Dimension) lattice measured by STM for the submonolayer TCNQ film is 8.22 Å × 12.49 Å, but, the 2D lattice for a monolayer slab of a 3D TCNQ (020) orientation, is 8.22 Å × 8.91 Å and with an angle of β = 98.54°. This is clearly different in one of the surface “inplane” axis length. This difference in the structure could be due to a lattice change induced by the molecule to substrate interactions. In the first monolayer the TCNQ molecules are lying flat on top of the Cu surface maximizing the number of bonds between the N and Cu while in the bulk TCNQ planes the molecules exhibit a zigzag arrangement (see Figure 3B). That means that the first layer seen by STM is the result of the flattening of the zigzag TCNQ planes structure resulting in a slightly larger lattice constant. In fact, taking into account this effect, the b lattice constant should go from 8.91 to 12.28 Å. The later value is very similar to that obtained by STM for the first layer (b = 12.49 Å). This interaction influence can be clearly inferred if we compare it with the STM results obtained for similar conditions but on Au(111) substrate by Fernandez et al.31 These authors have shown that the TCNQ molecules interact weakly with the gold metal underneath and are neutral. In this case, the arrangement of the TCNQ molecules on the Au-surface is equal to that predicted for a first layer of TCNQ with (020) orientation (Figure 2B), which is clearly different for our Cusubstrate case. We have discarded that this different arrangement could be due to a different TCNQ crystal orientation in the first TCNQ growth layer (i.e., (022) orientation in Figure 3C).

In Figure 4 we show a more detailed sketch of the differences of the 2D lattice for the first TCNQ monolayer covering and the 2D lattice of a monolayer slab of the monoclinic TCNQ(020) orientation. It seems that the TCNQ molecules at the surface forms 1D-like rows (in the vertical direction of the Figure 4) and they suffer a inter-row expansion and shift. Thus, it can be observed in the lateral view of the (020) TCNQ slab (Figure 3B) that the TCNQ molecules have an angle with respect to the substrate surface. Thus, the inter-row expansion in the measured STM image with respect to this model can be due to the fact that, in this case, the molecules are laying down more parallel to the substrate surface. The inter-row shift seems to happen because in this final configuration two TCNQ molecules belonging to two different TCNQ rows can share one Cu atom. These TCNQ−Cu−TCNQ interactions seems locally similar, but lightly distorted, to the local configuration present at the orthorhombic CuTCNQ phase I structure.32 However, we can discard the formation of a phase I CuTCNQ in the first monolayer. In equivalent growth conditions, the CuTCNQ has a (100) orientation33 and, in this orientation the TCNQ molecules of a molecular layer would form a perpendicular pattern, which is clearly not observed. Thus, we have shown that there is a clear distortion in the first monolayer of TCNQ on Cu(001) due to the strong TCNQ to Cu-metal interactions. This 2D structure adopted by the TCNQ molecules in the first monolayer, which is mainly given by the maximization of the Cu:TCNQ interactions, acts as pattern favoring the stacking of the TCNQ molecules in the further grown layer with the monoclinic (020) orientation, which is clearly a lightly relaxed structure of this one. However, it is still not known if this distortion only happens in the submonolayer regimen and relaxes at overmonolayers coverage thickness due to the interlayer molecules interactions or, if this difference is preserved and is always present at the Cu−TCNQ interface. At low coverage the measured XRD pattern (not shown here) only exhibits a Bragg peak at QT = 1.945 Å−1 at low perpendicular momentum (ls = 0.2). At very low perpendicular momentum we are mainly sensitive to the parallel to the surface plane structure. Although it is impossible to assign this to a structure with only one Bragg peak, it is possible to see which are compatible with it. This peak is compatible with both, the orthorhombic 2D lattice measured by STM (2D Miller index (5 2)), and the monoclinic 2D lattice coming from the bulk 26893


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Figure 5. Photoemission spectra (C 1s in the left panel and N 1s in the right panel) for a 0.4 and 17.1 ML TCNQ films (yellow and blue points respectively). The corresponding spectra for a very thick CuTCNQ film has been included as a reference (black line). The two colors used for the C (gray and green) in the TCNQ molecule sketch represent to the two observed C 1s peaks.

two types of carbon in this molecule. Lidquist et al.34 determines that these peaks are related to the C close to the cyano group and the in-ring C respectively. The difference in these two C-groups is due to the charge distributions in the TCNQ molecule.37 The satellite is a shakeup feature and is shifted 2.6 eV with respect to the 286.1 eV peak (like the highbinding energy satellite in N 1s) and is therefore attributed to an intramolecular electronic excitation. Thus, no satellite associated with the 284.8 eV peaks is observed. Therefore, since only one of the cyano C 1s peak has shakeup, this indicates the HOMO and LUMO are located near the cyano ends of the molecule. The growth of TCNQ molecules over the substrate on the 17.1 ML thick film has been studied by the appearance of a photoemission peak in the N 1s and C 1s energy region (Figure 5). When we compare the N 1s spectrum in the submonolayer sample (0.4 ML) with the very thick sample (17.1 ML) it can be clearly set that there is a peak shift toward higher binding energy. An equivalent shift is also observed in the C 1s spectrum of the submonolayer TCNQ film compared with the very thick film (see left panel in Figure 5). The submonolayer sample shows a main peak at 398.3 eV in the N 1s spectrum. This peak position is the same as that measured for a wellcharacterized CuTCNQ film, 19 ML thick growth on Cu(001) substrate as can be appreciated in the same figure where we have included this spectrum as a reference.33 In the very thick TCNQ sample, the main peak position appear at 398.9 eV. A peak shift equivalent to this has already been observed in other compounds where a Cu−N bond is formed.39 All the peaks (N 1s and the two C 1s peaks) have an average shift of 0.6 ± 0.1 eV. Thus, it can be said that there is a strong interaction between the TCNQ molecules and the substrate Cu atoms in the first formed layer. This fact is compatible with the difference in the 2D structure observed by STM in the submonolayer sample previously discussed in this paper. The small peak present in the N 1s spectrum at lower binding energy with respect to the main peak in the submonolayer thick film is attributed to defects present at the surface of not welldefined origin but that can be associated with C−N bonds with more saturated bonds by the acceptation of electrons by the

TCNQ (020) oriented (2D Miller index (24) and/or (0 5)). This peak remains at higher TCNQ coverage because it corresponds to (204) and/or (0 0 5) Bragg peak in the 3D monoclinic TCNQ structure. Thus, at coverage above the monolayer, the TCNQ grows with a bulk monoclinic TCNQ phase as we have already show. The interface layer directs this crystalline growth for coverages greater than the monolayer acting as a template layer. Growth of TCNQ on other less interacting substrates as Au(111)31 results in layers with the bulk TCNQ planes. In our case the interaction of TCNQ with Cu substrate is strong and the formation of such interface is preserved even at high coverages. The interface structure is close enough to the TCNQ(020) ones to allow the crystalline growth of the later acting in some way as a template for the TCNQ crystal growth. 3.II. Characterization and Modeling of the TCNQ Thin Film Electronic Structure. Figure 5 (right panel) shows N 1s core level spectra of TCNQ−film (blue points). The spectrum for a coverage of 17 ML is quite similar to those shown in literature (Lindquist,34 Grobman et al.,8 and Ikemoto et al.12). The spectrum has a most intense peak and a less intense second peak shifted 2.6 eV to a higher binding energy from the main one. This second peak has approximately 20% the intensity of the main peak. This relatively small peak is common to the core level spectra of TCNQ and is attributed to an intramolecular electronic excitation (shakeup) process.12,34−36 This process often occurs due to a sudden change in shielding felt by the valence electrons upon core photoionization.37 Aarons et al.36 noted that the only transition giving significant intensity involves the highest energy doubly filled orbital of TCNQ and the lowest energy unfilled valence orbital of the ion. The energy shift of the satellite from the main peak is related to the transition energy between the lowest unoccupied molecular orbital (LUMO) of the photoion and the highest occupied molecular orbital (HOMO).38 C 1s spectrum of the TCNQ film (blue points in Figure 5 (left panel)) is again similar to C 1s spectra previously published.12,34,35 The spectrum is formed by a double peak at 286.1 and 284.8 eV and a low-intensity satellite at higher binding energy. The double C 1s peak indicates the presence of 26894


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Figure 6. He I (A) and He II (B) UPS spectra of the different TCNQ film thickness compared to the fresh substrate (red line) and to a well characterized thick CuTCNQ film (black lines) used both as references. In the lateral inset of part A, we show a detail of the UPS spectra at the Fermi edge proximity. It is necessary to note that in this case we have placed the CuTCNQ reference spectrum at the bottom for clearness.

Figure 7. Local density of states (brown line) calculated for 1 layer of TCNQ on Cu(001) and bulk TCNQ (left and right panel respectively) compared to the corresponding experimental spectra (points). The calculations were corrected by an exponential background and the integral background. In the thinnest sample, Cu signal coming from the substrate has been included. The final intensities are shown by the corresponding color solid line.

cyano groups.39,40 This could be related to the Cu associated with only one cyano-group (see right panel of Figure 4). The C 1s and N 1s core level peaks, have been fitted, after subtraction of a Shirley background, with a Doniach-Sunjic combination of Lorentzian and Gaussian lineshapes.41 Assuming that the films are strictly homogeneous within the escape depth of the electrons, the ratio of the intensities of the C 1s and N 1s peaks is related to the atomic density ratio (XC/XN) by XC/XN = AIC1s /IN1s

In Figure 6, we show the He I and the He II UPS spectra of three TCNQ films. We have included the spectra corresponding to the Cu(001) and a thick CuTCNQ film growth on Cu(001) that will be used as references (red and black line respectively). In the thinners films (0.4 and 1.8 ML) the Cu substrate signal is clearly observed, but in the thicker film (17.1 ML) the substrate signal is not observed. This means, that in this film its thickness is higher than the technique mean free path sensibility and also that the substrate covering is quite homogeneous. In the thickest TCNQ film, the lack of electron emission at the Fermi level indicates the insulator character of the TCNQ. Besides this, the huge difference that exist between the 0.4 ML thick sample and the 17.1 ML sample spectra (yellow and blue points respectively) is noticeable. In the submonolayer sample, there is a bump around −6.45 eV in the He I and two features at −8.3 and −6.3 eV in the He II which are absent in the thickest TCNQ film (black arrows are used as an eye guideline). However, these peaks are quite similar to the

(1)

where A = (1/SC)/(1/SN) and SC and SN are the atomic sensitive factors determined for the pure chemical elements for the specific electron analyzer used.42 The average stoichiometry of the film calculated using this method is a C/N of 3.0 ± 0.1, which fits perfectly with the TCNQ formula (C12H4N4). 26895


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The Journal of Physical Chemistry C peaks that are measured in the CuTCNQ film used as a reference. This fact strengthen the idea of the strong interaction between the first layer of TCNQ molecules with the Cu atoms of the substrate surface. In the 1.8 ML thick case, a clear contribution of a pure TCNQ UPS spectra appears. In the lateral inset included in Figure 6A, we show the He I UPS spectra in the Fermi level vicinity. The fresh Cu has electrons at the Fermi and, the thick CuTCNQ film used as reference has a valence band edge located at 0.137 eV below the Fermi level. This is characteristic of its semiconductor behavior and is compatible with the band gap reported in literature.32 The TCNQ films are insulator when they are very thick. However, when the TCNQ film thickness is close to a monolayer, they exhibits an anomalous big density of state at the Fermi level (even higher than the Cu-metal), although this does not imply an increase in the conductivity of this interface monolayer. This higher density of electrons at the Fermi level is related to the strong Cu to TCNQ interactions at the metalfilm interface. Furthermore, the strong metal−TCNQ interaction in the first monolayer can also be observed in the strong distortion observed in both spectra (He I and He II) in the Cu d electronic bands (from 2.3 to 3.3 eV range of binding energy). In order to have a clear qualitative picture about the role of both TCNQ and Cu to the UPS spectra, we have calculated the electronic structure using DFT calculations obtaining a charge transfer toward the TCNQ LUMO,.43 The resulting density of states (DOS) of a TCNQ {010} slab over Cu (001) is shown in Figure 7. The shadowed area corresponds to occupied electronic states and the dashed lines to unoccupied states. The calculated gap is much smaller than the observed valence band edge, as it is expected for the calculation method which systematically underestimate the band gaps. The DOS curves have been shifted by 0.5 and 2.4 eV positive binding energy for the TCNQ monolayer on Cu and the bulk TCNQ respectively. There is an overall agreement between theory and measurements (see Figure 7). A very significant fact that can be observed in despite of the characteristics features of the spectra already mentioned, is the fact that in the close to monolayer samples there is an observable change in the low binding energy side at the vicinity of the Cu d band. The DOS calculated for one layer of TCNQ on Cu indicated that it corresponds to a bending of the Cu d orbitals which arise from the Cu−TCNQ interactions. This phenomenon can be related to the band bending observed in the Cu3N case.39 In both cases the band bending is produced by the injection of electrons at the Fermi level of the surface Cu atoms to the TCNQ LUMO level. In the Cu3N case the authors have shown that this band bending was also the origin of the peak shift observed in the N 1s photoemission peak. In fact, the N 1s shift shown in the Cu3N case for the authors is similar to the shift here reported. In the 1.8 ML film the UPS has a clear contribution of a TCNQ spectra but there is also a clear change at the vicinity of the Cu d band which is indicative of a strong Cu−TCNQ layer presence. This Cu−TCNQ layer remains at the Cu to TCNQ film interface. We have already shown that all the peaks (N 1s and the two C 1s peaks) have an average shift of 0.6 ± 0.1 eV in the submonolayer film with respect to a thicker TCNQ film. The fact that all the peaks are shifted can be related to a charge displacement from the Cu substrate to the TCNQ molecules of the first layer. This fact can be at the origin of the higher

density at the Fermi level as can be observed in the inset in Figure 7A. It is interesting to notice that in the 1.8 ML thick sample, the UPS has features that correspond to the bulk TCNQ but in a detailed inspection of the spectrum (overall at the Cu d band energies) a contribution of one TCNQ layer with strong Cu interactions can be deduced. This result seems to indicate that the TCNQ−Cu interaction layer is always present at the TCNQ−Cu(substrate) interface.

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CONCLUSIONS Our studies of the TCNQ thin films grown on Cu(001) substrate have shown that the film is a well-crystallized monoclinic single phase where the TCNQ molecules are stacked with respect to the substrate surface with a given orientation. Thus, the resulting film is very textured with a TCNQ (020) orientation except in the TCNQ−substrate interface which exhibits a molecular reordering due to the strong Cu−TCNQ interactions. This well-ordered interface layer between the TCNQ and the Cu substrate directs the formation of this well crystallized TCNQ film, acting in some way as film patterning. A well-ordered film of TCNQ open the field for technological applications of this semiconducting material, in which amorphous or polycrystalline structures could imply a big number of defects that can alter their conduction properties. However, the strong electron acceptor character of the TCNQ produces a strong interaction with the Cu surface atoms, resulting both in a change in the TCNQ molecular arrangement on the first overlayer and, a change in the electronic structure of this first overlayer. The strong TCNQ−Cu interaction at the TCNQ film to substrate interface changes the density of state at the interface. The measured changes associated with the Cu d-band in the UPS spectra for different film thickness indicates that this Cu− TCNQ modified monolayer is always present at the interface. In addition to the changes in the d states at the valence band, an increase of the density of states at the Fermi level is observed evidencing an electron injection from the substrate to the empty states of the Cu−TCNQ. The changes associated with the evolution of the valence band at the interface have been studied with ab initio DOS calculations. These calculations can reproduce the changes in the density of states associated with the evolution of d-states as well as the empty density of states at the Fermi level thus confirming the strong electron acceptor character of the TCNQ. This electron injection can locate charges close to the Fermi level at the interface which result in a local bandbending at the interface similar to the band bending previously reported at the interface of copper nitrides compounds. This band bending at the interface could play an important role in the carrier injection in the semiconducting TCNQ layer.

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AUTHOR INFORMATION

Corresponding Authors

*(M.J.C.) E-mail: [email protected]. *(J.A.) E-mail: [email protected]. ORCID

M. J. Capitán: 0000-0002-6576-4365 Notes

The authors declare no competing financial interest. 26896


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ACKNOWLEDGMENTS This work was supported by the Spanish CICyT under Grant No. MAT2013-47869-C4-3-P. J.I.B. would like to thank the Abengoa Research Center under “The Virtual Material Design Project” (2nd stage) for his financial support. Parts of this research were carried out at the light source MAX-lab IV and Hasylab at DESY, a member of the Helmholtz Association (HGF). We would like to thank Dr. O. Seeck his for assistance in using beamline W1 and Dr. S. Carlson for assistance in using beamline I811.

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ABBREVIATIONS

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REFERENCES

TCNQ, 7,7,8,8-tetracyanoquinodimethane; UHV, ultra high vacuum; ML, monolayer; XRD, X-ray diffraction; XPS, X-ray photoelectron spectroscopy; UPS, ultraviolet photoelectron spectroscopy; DFT, density functional theory; 2D, 2 dimension; STM, scanning tunnel microscopy; LUMO, lowest unoccupied molecular orbital; HOMO, highest occupied molecular orbital; DOS, density of states

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TCNQ Grown on Cu - American Chemical Society

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