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Optical and electronic properties of graphene nanoribbons upon adsorption of ligand-protected aluminum clusters† Claudia Gomes da Rocha,*a P. Andre Clayborne,b Pekka Koskinena and Hannu Ha¨kkinenab We have carried out first-principles calculations to investigate how the electronic and optical features of graphene nanoribbons are affected by the presence of atomic clusters. Aluminum clusters of different sizes and stabilized by organic ligands were deposited on graphene nanoribbons from which the energetic features of the adsorption plus electronic structure were treated within density-functional theory. Our results point out that, depending on their size and structure shape, the clusters perturb

Received 5th September 2013, Accepted 2nd December 2013

distinctively the electronic properties of the ribbons. We suggest that such selective response can be

DOI: 10.1039/c3cp53780c

characterization medium of atomic clusters. In addition, we demonstrate that atomic clusters can fine-

measured through optical means revealing that graphene nanoribbons can work as an efficient tune the electronic and spin-polarized states of graphene ribbons from which novel spin-filter devices

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could be designed.

1 Introduction Graphene,1 the first two-dimensional atomic crystal composed of carbon atoms arranged in a honeycomb grid, has been responsible for numerous technological and scientific breakthroughs.2 Depending on the particular purpose of the device or application, graphene can be tailored in distinct circuit frameworks and, consequently, will be in contact with different sorts of bulky materials, contact leads, multilayer systems or substrates. In particular, the implementation of graphene in certain devices such as field-effect transistors,3 non-volatile memory elements,4 or logical switches5 can become rather challenging since its electronic structure is characterized by the absence of an energy band gap. Doping or adsorption processes, when performed in a controllable way, work as an efficient strategy for modulating the band gap energy of graphene6–8 or its structure.9 The monitored deposition of atoms or molecules on graphene affects its local carrier concentration leading to a gradual change in its electronic properties. In this sense, graphene demonstrates to be a versatile two-dimensional platform capable of interacting with a broad hierarchy of materials ranging from simple atoms to

a

¨skyla ¨, Nanoscience Center, Department of Physics, University of Jyva ¨skyla¨, Finland. E-mail: [email protected] 40014 Jyva b ¨, Nanoscience Center, Department of Chemistry, University of Jyva¨skyla ¨skyla¨, Finland 40014 Jyva † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c3cp53780c

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bulky structures. Bridging these two tiers, one finds another class of materials that manifest fascinating hybrid properties between atomic and bulky versions of the matter: the atomic clusters.10–12 Atomic clusters exhibit intriguing properties which are drastically correlated with their size and core morphologies. Regarding their field of applications, the inherent high surface-to-volume ratio exhibited by these systems makes them ideal elements for catalysis processes. They have been widely used as catalysts in several industry sectors including pharmaceuticals, oil refining and fuel cells. Other application examples include: (i) biomedical imaging and clinical diagnoses can benefit from the proper manipulation of the photoluminescent wavelength of platinum nanoclusters;13 (ii) the size-dependent nonlinear optical characteristics of gold clusters were found to be superior for optical power limiting applications in comparison to plasmonic gold nanocrystals;14 (iii) gold nanoclusters embedded in graphene nanocomposites have been successfully probed as therapeutic materials against cancer cells.15 Besides the applicable opportunities that these materials offer, they have recently rendered a contribution of enormous academic and scientific relevance. The first experimental observation of the quantum phenomenon known as the atomic collapse state was achieved by gradually piling up calcium dimers over a graphene surface.16 Scanning tunneling microscopy measurements were conducted to capture such unconventional states which were theoretically predicted (many years ago) to occur around a super-heavy atomic nucleus. The last two experimental realizations mentioned previously indicate that the combination of such exceptional materials

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(clusters and graphene) can result in captivating new physical phenomena and ultimate technological innovations. Numerous theoretical contributions therefore have devoted their efforts to determine how the intrinsic properties of these materials are affected when they interact and to unveil the main mechanisms which mediate their adsorption process. Density-functional theory (DFT) has been employed to systematically analyze the adsorption of beryllium atoms and clusters on mono- and bilayer graphene.17 It has been found that Be clusters bind stronger to bilayer than to monolayer graphene and that their stability on the surface is improved as the size of the clusters increases. DFT has also been used to model the adsorption of palladium clusters on graphene.18 There, one has pointed to the possibility of improving the hydrogen storage capacities of graphene by adsorbing Pd clusters on its surface. In another important contribution, one has demonstrated that the catalytic properties of platinum clusters can be tuned by applying isotropic strain on the supporting graphene platform.19 Finally, to control the electronic and magnetic properties of graphene by depositing Co13 clusters on its surface has been proposed.20 All these contributions have aimed at investigating the adsorption processes of bare atomic clusters on graphene. The atoms of the clusters directly exposed to the surroundings can be very reactive and therefore their fundamental quantum state can be rather sensitive to external perturbations. A solution to this problem can be, for instance, to adsorb atomic clusters which have their main core protected by ligands. Good candidates for this task comprise a series of intriguing cluster systems composed of a metallic core surrounded by organic ligands. The properties and stability of many of these atomic complexes have been found to be consistent with the superatom model, an alternative electron counting scheme widely employed to understand the electronic structure of bare and ligand-protected clusters.21 In this work, we present a systematic theoretical study about the adsorption processes of ligand-stabilized aluminum clusters22,23 on graphene nanoribbons (GNRs). First-principles methods were employed to analyze the electronic and optical properties of the pristine and adsorbed graphene platforms. We exposed zigzag and armchair graphene nanoribbons to a series of representative aluminum-based complexes of distinct sizes and distinct geometries. From the analysis of the electronic structure and optical response of GNRs, we find that they can work as a promising characterization tool for atomic clusters since each foreign object affects distinctively the electronic features of the ribbons.

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Fig. 1 Atomic structure of the hosts: (a) 18-AGNRs and (b) 8-ZGNRs. The translational direction of the ribbons is set along the ˆx-axis and they are modeled in a rectangular supercell of dimensions (a) 34.2  60  60 Å3 and (b) 29.5  60  60 Å3, respectively. The dashed circles show where the adsorbates were initially placed: (C) center or (E) edge of the ribbons.

armchair-(zigzag-)edge nanoribbon containing N carbon atoms along its width (N zigzag-rows). The structures are modeled in a rectangular supercell, with its translational direction along the xˆ-axis. The lengths along xˆ were chosen to be large enough to avoid interaction between periodically repeated image-clusters. The dangling bonds of the carbon atoms located along the edges are saturated with hydrogen atoms. A summary of all adsorption elements investigated in this work is presented in Fig. 2. The width of ribbons was chosen to be comparable with the size of the full cluster, Al4(C5H5)4. The calculations for the isolated ribbons and adsorption processes were performed via density-functional theory (DFT) implemented within the localized-basis set DFT code SIESTA package.24,25 Generalized gradient approximation (GGA-PBE)26 was chosen as exchange– correlation potential for all adsorption events except for the case of the full cluster Al4(C5H5)4 where non-local exchange– correlation functional which includes van der Waals interactions (vdW-DF) was adopted. In such prototypical p–p stacking formed between the organic ligand and the ribbon, van der Waals forces can play an important role in the description of such non-covalent interactions. We have chosen the functional reported by Dion et al.27,28 that has been widely used to predict structural and electronic properties of molecules and solids

2 Host systems and computational methods We begin by introducing the pristine armchair (AGNRs) and zigzag (ZGNRs) graphene nanoribbons which will host the atomic clusters. Their structures are shown in Fig. 1. A ribbon can be specified by the number of atoms arranged along its width. We use the notation N-AGNR (N-ZGNR) to refer to an

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Fig. 2 Schematics of the optical apparatus which will perceive the adsorption processes of (i) an aluminum atom, (ii) AlCp, (iii) Al4Cp3, and (iv) Al4Cp4 (Cp = C5H5) on graphene nanoribbon hosts.

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in which long-range vdW forces should not be neglected.29 Norm-conserving pseudopotentials with core corrections30 and a split-valence double-z basis (DZP) of pseudoatomic orbitals with an orbital confining energy of 0.05 eV were used, and the value of the energy cutoff was 150 Ry. The structural relaxation was performed within the conjugate gradient algorithm under 10  1  1 G-centred k-mesh and with a maximum force tolerance of 0.05 eV Å1. The electronic structure of the ribbons was then obtained in a sampled G-centred Brillouin zone with a grid of 52  1  1. The same parameters described above were adopted to conduct spin-polarized calculations for the zigzag ribbon as a host31 but relativistic corrections30 were included in the norm-conserving pseudopotentials in this case. To investigate the optical properties of the systems, we also employed the SIESTA method24,25 which adopts dipolar approximation to compute the real and imaginary parts of the dielectric function, e(o) = e1(o) + ie2(o), Ephoton =  ho being the energy of the incident radiation. We set an energy range of [0, 6] eV for the photon absorption, Gaussian broadening of 0.03 eV and a k-mesh of 102  1  1, which represents that the optical polarization vector is oriented parallel to the translational direction of the ribbons. This description does not take into account many-body effects32–34 which are proven to play an important role in the energy position of the optical absorption peaks. Nonetheless, our main goal is to deliver a qualitative demonstration of a physical effect that can serve as a characterization mean of adsorbates – atomic clusters in particular – placed over graphene ribbons. The optical properties of graphene nanoribbons exhibit remarkable features due to their peculiar one-dimensional physics. They naturally have an anisotropic structure which renders an anisotropic optical response with respect to the polarization of the incident photon beam. The selection rules which governed the absorption spectra of nanoribbons have already been unveiled and extensively investigated by several authors who also opted to follow distinct theoretical approaches, e.g. single p-band tight-binding approximation,35–37 Pariser–Parr– Pople-model Hamiltonian,38 and DFT.39 Basically the dipolar selection rules of AGNRs under a light wave polarized parallel to its longitudinal direction involve optical transitions between subbands with same angular momentum, l.35,36 In other words, intense transitions commonly labeled as Sll can occur at the G point between subbands lv 2 lc, where Dl = lc  lv = 0 and lv (lc) is the angular momentum associated with the valence (conduction) bands. The lowest energy transition S11 provides a direct reading of the energy band gap of semiconducting ribbons. In ZGNRs, these selection rules exhibit distinct criteria. For a nonmagnetic ZGNR, the optical transitions occur only between subbands with the same parity35,36 and their characteristic edge states also bring an important contribution to the optical absorption. While assuming spin-polarization in the electronic description of ZGNRs, however, transitions between subbands of different parities can also take place38,39 and the optical excitations get more difficult to identify. It is not the scope of this work to perform a detailed analysis of the optical selection rules in GNR structures which are highly anisotropic. From such anisotropic

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optical response, Gundra and Shukla have suggested that optical probes could be applied to oriented GNR samples as a means of structural characterization.38 Similar optical characterization schemes could be therefore extended to GNR hosts exposed to adsorbates. The results obtained for each adsorption process as shown in Fig. 2 will be discussed in detail in the following sections.

3 Results and discussion The main adsorbate element of this work is the ligand-stabilized aluminum cluster, Al4Cp4 (Cp = C5H5), a cluster unit which belongs to D2d point-group symmetry and obeys the superatom rule where the number of electrons in the cluster core has a correspondence with the jellium shell model. Its first synthesis can be found in the work of Dohmeier et al.22 and a detailed analysis about the electronic structure of isolated Al4Cp4 and other ligand-protected clusters can be found in the works carried out by Clayborne et al.23 and Lopez-Acevedo et al.40 The cluster is formed by an Al4+4 core where each of its aluminum atoms loses charge to the Cp ligands and its HOMO–LUMO gap was found to be 3.43 eV. Its electronically stable structure contains 8 superatom electrons and those states are combinations of the Al(sp) valence electrons. The other adsorbates considered here are sub-units removed from the full Al4Cp4 cluster. One extra adsorption example involving an Al4+4 cluster surrounded by organic permethylated ligands is presented in the ESI.† The objects are positioned approximately at the center of the ribbon or on one of its edges (cf. Fig. 1) and the atomic arrangements which rendered the lowest energy state after the relaxation process are depicted in Fig. 3. We begin by defining a series of adsorption quantities and structural parameters derived from our calculations presented in Table 1. The binding energy Eb is defined as Eb = Ec + Er  Ecr,

(1)

Fig. 3 Lowest energy atomic structures of the 18-AGNR plus (a) Aluminum adatom (center), (b) AlCp (edge), (c) Al4Cp3 (center), and (d) Al4Cp4 (center) obtained after relaxation with Cp = C5H5. The bottom insets zoom on the aluminum tetrahedron core of (left panel) Al4Cp3 and (right panel) Al4Cp4 interacting with the host. Distances are reported in angstroms.

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Table 1 Adsorption quantities calculated for each adsorbate deposited on the 18-AGNR: binding energy (Eb), charge transfer (Dq), the average distance between the respective cluster unit and the host (d), the standard deviation (sz) associated with the average z-coordinates of the host atoms, and the total energy difference between center and edge adsorption (DECE)

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Adsorbate

(i) Al

(ii) AlCp

(iii) Al4Cp3

(iv) Al4Cp4

Center adsorption 1.28 Eb (eV) Dq (e) 0.93 D (Å) 2.25 1.19  102 sz (Å)

0.08 0.27 3.60 0.11

1.01 1.00 2.61 0.13

0.46 0.17 3.20 8.71  102

Edge adsorption 1.30 Eb (eV) Dq (e) 0.90 D (Å) 2.16 3.28  102 sz (Å)

0.11 0.21 3.59 0.12

0.76 0.95 2.44 0.16

0.28 0.08 3.26 0.13

DECE (eV)

0.03

0.12

0.06

0.00

where Ecr is the total energy of the adsorbed nanoribbon (‘cr’ stands for ‘cluster + ribbon’), Ec is the total energy of the isolated cluster, and Er is the total energy of the isolated nanoribbon. In order to avoid basis set superposition errors, the Eb value was amended with counterpoise corrections.41,42 Another important value to compute is DECE = EC  EE which corresponds to the total energy difference between center (EC) and edge (EE) adsorptions. This quantity provides information about the location-selectivity of the adsorption process. Charge transfer between the adsorbate and the host, Dq, is defined as Dq = q  q 0

(2)

where q is the Mulliken population of the isolated ribbon and q 0 the ribbon’s Mulliken population with an adsorbate. Electron transfer to the ribbon yields Dq o 0, and from the ribbon produces Dq > 0. For this calculation, we used the minimal basis set of SIESTA, single-z (SZ), and their former relaxed positions extracted from the ribbon + cluster calculations. It is worth mentioning that Mulliken charges are rather dependent on the choice of the basis set. Therefore we also performed Bader charge analysis43 to support Mulliken calculations. The results found in the ESI† confirm that both methods are in agreement. One can also find results for the charge-density difference in the adsorbed systems in the ESI.† Finally, we have obtained the average distance along the ˆz-axis between bottommost atoms of the cluster and the host unit (d) and the average z-coordinates of the host atoms (hz) together with its standard deviation (sz). The latter provides a measurement of how much the graphene ribbon is distorted in relation to its perfect flat form at the z = 0 plane. 3.1

incomplete cluster, Al4Cp3, since one of its Al atoms does not have a ligand pair. These two aluminum-based units adsorb preferentially at the center of a hexagon. Nonetheless, Al4Cp3 binds stronger at the center of the ribbon whereas the adsorption location for the adatom (edge or center) does not play a role. The Al4Cp3 cluster also yields a pronounced deformation on the ribbon, confirming that its more complex atomic structure can impact effectively the adsorption process. On the other hand, the geometry of the cluster is also disturbed with respect to its original isolated form. The bottom panels of Fig. 3 show the relaxed structures of the metallic core (ligands are omitted) for both Al4Cp3 and Al4Cp4 units interacting with the graphene host. For Al4Cp3, one can observe that the base of the tetrahedron core is enlarged and the distance between the base and the apex is reduced, whereas the regular tetrahedron skeleton of the complete Al4Cp4 cluster is preserved. The latter also has one of its pentagonal organic ligands preferentially relaxed on top of a hexagonal ring of the ribbon but its 3-fold symmetry axes are staggered with respect to ˆz. Following opposite interaction trends, AlCp repels the ribbon host as a result of a more substantial charge transfer from the Al to the cyclopentadienyl ligand than to the graphene. For this reason, AlCp prefers to accommodate on the ribbon edge since this region is more propitious to bend (cf. Fig. 3). Finally, as expected, the full cluster interacts with the graphene ribbon via van der Waals forces, which can be deduced from the calculated average distances. The cluster is more favorable to adsorb on the center than on the edge of the ribbon where notable local bending can be noticed. Still, the relative low binding energy values obtained for this cluster indicate that it simply physiorbs on the ribbon platform and therefore it is highly prone to diffuse along the host channel due to thermal effects. We now analyze how each adsorption process perturbs the electronic properties of the 18-AGNR ribbon. Fig. 4 and 5 show, respectively, the energy dispersion relations and DOS versus energy results for the pristine and center-adsorbed cases. DOS plots were generated by using 0.05 eV as a broadening

Armchair-edge graphene nanoribbons as a host

One can see from the data of Table 1 that the aluminum adatom and Al4Cp3 are the stronger adsorbates. This can be attributed to the significant charge transfer amount of approximately 1e that they impart to the host. As previously shown by Chan et al., Al donates its outermost valence electron, at the 3p shell, to the graphene surface.44 The same occurs for the

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Fig. 4 Band structures obtained for the (0) isolated 18-AGNR and upon center adsorption of (i) Al atom, (ii) AlCp, (iii) Al4Cp3, and the full cluster (iv) Al4Cp4. The Fermi energy is set at 0 eV and it is marked by dashed lines. The k-vector interval spanned in the band structure is [0, p/a] or (G, X) a being the longitudinal length of the supercell.

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Fig. 5 Density of states (DOS) versus energy obtained for the (0) isolated 18-AGNR and upon center adsorption of (i) Al atom, (ii) AlCp, (iii) Al4Cp3, and the full cluster (iv) Al4Cp4. The Fermi energy is set at 0 eV and it is marked by dashed lines.

parameter for the energy eigenvalues. Although the chemical composition of all adsorbates is the same (Al, C, and H atoms), they portray distinct geometric forms perturbing the electronic structure of the ribbon in a selective manner. The modulation of its electronic structure, therefore, is rather susceptible to the adsorbate atomic features since quantization effects due to size reduction are prominent. The object which less impacts the electronics of the ribbon is AlCp. One can see from panel (ii) of both figures that the energy gap of the ribbon (Eg E 0.3 eV) and all other subbands remains constant with respect to the pristine case. One can only observe the appearance of a flat energy level (or a peak in the DOS plot) at E1 eV associated with the molecule. As a consequence of the significant carrier transfer that adsorbates (i) and (iii) render to the ribbon, the Fermi energy shifts upwards indicating that ribbon states were occupied. In addition, numerous band splittings result from such physical interaction. Although these adsorbates are not capable of provoking evident mechanical distortions on the ribbon surface, their relevant charge transfer effects alter the carrier density of the ribbon, creating then an effective electronic offset which affects the overall electronic response of the host.45 Finally for the full protected aluminum cluster, we observe the appearance of three flat energy states related to the type P superorbitals of the cluster below the Fermi energy. One of these superatom states lays inside the ribbon’s energy gap that is now enhanced to Eg E 0.4 eV. Such an occupied state makes the combined system ribbon + cluster behave like electron donors. The Al4Cp4 adsorption is characterized by a non-covalent functionalization, which is not capable of provoking significant charge transfer. Still, the electronic structure of the ribbon is rather affected by the full cluster due to its larger structure size, i.e. a greater extent of orbital hybridization between the adsorbate and the host can take place. This was also verified for other cluster systems such as carbon and boron fullerenes deposited on graphene surfaces.46 Results for edge adsorption are not shown here since similar overall trends in the electronic structure of the ribbons were found.

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Fig. 6 Imaginary part of the dielectric function (e2) versus energy of the incident photon ( ho) assuming an incident light wave polarized parallel to the longitudinal direction of the 18-AGNR hosting a particular cluster unit adsorbed on its central area. The curves were shifted upwards with respect to the optical spectrum of the isolated ribbon for better visualization. The figure is organized as: adsorbates which induce (a) large and (b) small charge transfer.

The perturbations caused by the adsorbates on the ribbon electronic properties were visualized through optical absorption spectra. Fig. 6 and 7 show the results for center and

Fig. 7 Imaginary part of the dielectric function (e2) versus energy of the incident photon ( ho) assuming an incident light wave polarized parallel to the longitudinal direction of the 18-AGNR hosting a particular cluster unit adsorbed on its edge. The curves were shifted upwards with respect to the optical spectrum of the isolated ribbon for better visualization. The figure is organized as: adsorbates which induce (a) large and (b) small charge transfer.

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edge adsorptions, respectively. The peak associated with the ribbon energy gap is blueshifted for the full cluster deposited on the AGNR. The higher photonic excitations also respond to the cluster presence. However, there is no evident discrepancies in the optical responses while comparing the results for center and edge configurations. For the adatom and Al4Cp3 units, the lowest energy optical transition attributed to the energy gap is of course absent, but several optical excitations associated with the multiple band splittings can be observed. One can also notice a significant disparity between the results of center and edge adsorptions. These two adsorbates form a relatively strong interaction with host material and depositing them over one of the ribbon’s edge results in longitudinal symmetry breaking of the quantized states. Further band anti-crossings emerge from the electronic structure of Al or Al4Cp3 adsorbed on the ribbon’s edge (not shown), giving rise to a richer optical spectrum, as seen in Fig. 7. Finally, the optical response of the 18-AGNR under AlCp adsorption is practically not affected due to the repulsing nature of their interaction as discussed above. These results confirm that the cluster units affect distinctively the electronic and optical properties of the graphene nanoribbon, a prominent sensor platform from which information about the adsorbates can be extracted from its characteristic one-dimensional physics. 3.2

Zigzag-edge graphene nanoribbons as a host

Zigzag ribbons can portray distinct electronic characters depending on how their spin-edge states are oriented.47 It is well-known that the ground state of zigzag ribbons is found to be semiconducting when their two opposite edge-states are aligned antiferromagnetically (AF). In addition, their spin m and k-bands are degenerate. Nonetheless ZGNRs can also reveal a metastable metallic ferromagnetic (FM) state with no spin degeneracy. Fig. 8 depicts the electronic DOS for the particular zigzag ribbon host used in this study, the 8-ZGNR. An energy gap of 0.45 eV is found for the system under antiferromagnetic configuration whereas a metallic character emerges when its spin-edge states are arranged ferromagnetically. The total energy difference between the AF and FM configurations is dependent on the ribbon width, in our case we obtained B0.09 eV. It is worth mentioning that

Fig. 8 Density of states (DOS) per spin obtained for the isolated 8-ZGNR under (a) antiferromagnetic and (b) ferromagnetic edge-configurations. The Fermi energy is at 0 eV and it is marked by dashed lines.

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the stability of edge magnetism in zigzag nanoribbons is yet a topic under debate. Other types of edge formations such as edge-reconstruction,48 -passivation or -closure have also been indicated as possible stabilization mechanisms for ZGNRs.49 Still, evidence of edge magnetism in nanoribbons can be found in the literature. Joseph Joly et al.50 have successfully observed magnetic edge states in graphene nanoribbon samples by means of near-edge X-ray absorption fine structure (NEXAFS) and electron-spin resonance (ESR) measurements. The adsorption case studied in this part deals solely with the Al4Cp3 cluster on the 8-ZGNR with antiferromagnetic and ferromagnetic spin-edge orientations. The deposition takes place over the center and edge regions of the ribbon. The most stable structure is the (1) edge-adsorption of the cluster over the ribbon under antiferromagnetic alignment followed by (2) edge-adsorption over ferromagnetic configuration, (3) center-adsorption over ferromagnetic configuration, and finally (4) center-adsorption over antiferromagnetic configuration. The consecutive total energy differences are DE(1  2) = 0.07 eV, DE(2  3) = 0.52 eV, and DE(3  4) = 0.05 eV, revealing that the cluster preferentially accommodates on the ribbon edges. The average separations between the cluster and the ribbon found after relaxation of the structures are d1 = 2.25 Å, d2 = 2.25 Å, d3 = 2.65 Å, and d4 = 2.65 Å. The optical responses of the pristine zigzag ribbon and under center adsorption of Al4Cp3 are shown in Fig. 9. As previously verified, this cluster unit interacts rather strongly with the ribbon. As expected, a significant charge transfer from the adsorbate to the ribbon takes place and the system’s Fermi energy is shifted upwards. This charge transfer leads to the occupation of empty conduction bands of the ribbon located in the vicinity of the Fermi level. As a result, one can observe the appearance of low-energy optical transitions associated with the presence of the cluster. This cluster, therefore, demonstrates to be an effective energy modulator for the ribbon spin-states. Still, the cluster adsorption is not capable of breaking the spin-degeneracy of the bands for the antiferromagnetic case.

Fig. 9 Imaginary part of the dielectric function (e2) versus energy of the incident photon ( ho) assuming an incident light wave polarized parallel to the axial direction of the 8-ZGNR with edge-states oriented (a) antiferromagnetically and (b) ferromagnetically. The spectra are split into spin m and k-states. Dashed lines show the optical response of the pristine ribbon while dense lines correspond to the case where Al4Cp3 unit is adsorbed on the center of the ribbon.

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Fig. 10 Imaginary part of the dielectric function (e2) versus energy of the incident photon ( ho) assuming an incident light wave polarized parallel to the axial direction of the 8-ZGNR with edge-states oriented (a) antiferromagnetically and (b) ferromagnetically. The spectra are split into spin m and k-states. Dashed lines show the optical response of the pristine ribbon while dense lines correspond to the case where the Al4Cp3 unit is adsorbed on one of the edges of the ribbon.

It turns out that for center adsorption, the cluster perturbs equally the magnetization of the two edges. This scenario can be altered by placing the cluster on one of the ribbon’s edges. As demonstrated by Wang et al.,51 a spin-polarized profile in antiferromagnetic ZGNRs is not only achieved by the adsorption of magnetic objects on the ribbon. They successfully demonstrated that nonmagnetic atoms can also produce spin-polarized transport in antiferromagnetic zigzag ribbons as long as the magnetization symmetry of the edges is broken. This phenomenon can be observed by adsorbing the cluster on one of the ribbon edges [cf. Fig. 10(a)]. Clearly the optical spectra of both spin-states differ. In addition, their energy gap values decrease to 0.32 eV confirming that the cluster adsorption is capable of modulating the electronic character of the ribbon. Panel (b) contains the results for the ferromagnetic case where another interesting feature can be observed: an intense absorption peak at ho E 0.07 eV emerges for both spin-states associated with the opening of a small energy gap. To understand these distinct electronic scenarios established depending on the cluster position and the magnetic arrangement of the ribbon, we have calculated the total (TDOS) and the partial density of states (PDOS) projected on aluminum atoms [cf. Fig. 11]. If placed on the center of the host, the cluster donates its extra electrons shifting hence the Fermi level of the system upwards similarly to the armchair nanoribbon case studied previously. This can be seen in panels (a) and (c) of Fig. 11 which depict the results per spin-state for antiferromagnetic and ferromagnetic configurations, respectively. As a consequence of this charge transfer, a spin-degenerate valence-p state associated with the Al atoms appears at B1 eV (above the system Fermi level). When the cluster stands on one of the ribbon’s edges, the Al 3p peak splits and hybridizes with the nanoribbon states in the vicinity of the Fermi energy independently of the edge-magnetic arrangement as shown in panels (b) and (d) of the same figure. These findings demonstrate how important the edge-morphology of graphene nanoribbons is while dealing with adsorption processes. The same

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Fig. 11 Total Density of States (TDOS) and Projected Density of States (PDOS) on the cluster aluminum atoms for the adsorption of the Al4Cp3 unit on the 8-ZGNR. Curves with gray fillings correspond to the PDOS results while the remaining curves correspond to the TDOS. All the results are split into spin-m (black and blue curves) and k (brown and red curves) states. The Fermi energy is set at 0 eV and it is marked by dashed lines. The labels on the upper-left part of each panel refer to antiferromagnetic (AF-) or ferromagnetic (FM-) edge-configurations with the adsorbate placed on the center (C) or edge (E) regions of the ribbon.

cluster unit which prominently interacts with an armchair host via charge transfer can disclose other interacting mechanisms when adsorbed on zigzag ribbons.

4 Conclusions This study aimed at improving our understanding of how massive adsorbates such as Al cluster complexes interact with graphene-based structures and how they impact the electronic, optical, and magnetic properties of the hosts. We have demonstrated that each particular adsorbate affects the electronic structure as well as the optical responses of the ribbons in a selective manner. In other words, atomic clusters with their size-dependent features fine-tune the electronic and magnetic properties of the ribbons. If the cluster’s metallic core is directly exposed to the graphene host, their interaction can be amplified or reduced depending on the cluster core topology and nature of its ligands. As a result, the electronic structure of the ribbon will respond accordingly. When the cluster organic ligands intermediate the interaction with the host, a noncovalent functionalization takes place in which the molecular orbitals of the adsorbate hybridize with the low-energy p-orbitals of the ribbon. For the particular case of the Al4Cp4 cluster (Cp = C5H5), the adsorption transforms the graphene nanoribbon into an electron donor. These results indicate that graphene nanoribbons could act as an effective characterization medium of atomic clusters from which their information could be captured within optical measurements. By allowing other sorts of edge stabilization mechanisms on the ribbon hosts, e.g. edge-magnetism in zigzag nanoribbons, one can collect further signatures of the adsorbate. For example, Al4Cp3 – a cluster which is prone to adsorb on one of the ribbon edges – breaks the characteristic spin-state degeneracy of ribbons polarized antiferromagnetically. In cluster physics ‘‘each atom counts’’ in defining the properties of the aggregate. Low-dimensional carbon systems, therefore, appear to

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be ideal detectors of such atomic scale perturbations. It is worth mentioning though that there are many other experimental techniques, besides optical or fluorescence methods, that can also benefit from the high sensitivity features of lowdimensional carbon-based materials. Some examples are: carbon nanotube electronic transducers from which the minute dynamical conformation of protein molecules can be detected in real-time52 or graphene nanopore platforms used to scan DNA strands translocating through it.53

Acknowledgements We would like to acknowledge The Academy of Finland for sponsoring this research and the CSC – IT Center for Science in Finland for the computational resources.

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Phys. Chem. Chem. Phys., 2014, 16, 3558--3565 | 3565

Optical and electronic properties of graphene nanoribbons upon adsorption of ligand-protected aluminum clusters.

We have carried out first-principles calculations to investigate how the electronic and optical features of graphene nanoribbons are affected by the p...
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