Doping Effects in the Charge Transport of GrapheneâPorphyrins

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Doping Effects in the Charge Transport of Graphene-Porphyrins Gloria Ines Cardenas-Jiron, Yanara Figueroa, Narendra Kumar, and Jorge M Seminario J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b08624 • Publication Date (Web): 12 Jan 2016 Downloaded from http://pubs.acs.org on January 17, 2016

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The Journal of Physical Chemistry

Doping Effects in the Charge Transport of Graphene-Porphyrins Gloria Cárdenas-Jirón1*, Yanara Figueroa1, Narendra Kumar2, Jorge M. Seminario2,3,4* 1

Laboratorio de Química Teórica, Facultad de Química y Biología, Universidad de Santiago de Chile (USACH), Casilla 40, Correo 33, Santiago, CHILE. 2 Department of Chemical Engineering, 3 Department of Electrical and Computer Engineering 4 Materials Science and Engineering Program Texas A&M University College Station, Texas, USA ABSTRACT Doping effects of B, N, and PHP in graphene and of aza N in porphyrin of grapheneporphyrin (GP) complexes bonded through an amide group have been studied using density functional theory (DFT) and time-dependent DFT mainly at the M06/6-31G(d,p) level of theory. Structural, optical, electronic, and charge transport properties are analyzed for a set of eight GP complexes.

We found that all GP complexes feature the characteristic

absorption bands of porphyrin in the visible region, Q and Soret, showing P→P electronic transitions. Charge transfer between the fragments graphene and porphyrin in both directions P→G and G→P is obtained in the excited states but the absorptions are of lower intensity when compared to the P→P ones. Doping only the graphene group of GP yields blue-shifted porphyrin absorptions; however, doping graphene and porphyrin yields redshifted absorptions. Doping leads to a reduction of the HOMO-LUMO gap in all GP complexes, because the dopant atoms perturbation of the electronic density favors the electronic transitions in the visible region. Depending on the dopant type and the direction of the bias voltage, introducing dopants can significantly affect the electrical conductance of graphene-porphyrin complexes. The results of this work show that porphyrin keeps its properties as photosensitizer when it is covalently bonded to graphene; therefore these complexes can be used as efficient harvesters of solar light. *authors to whom correspondence should be addressed; email: [email protected], Phone: 56-2-27181137 [email protected], Phone: 979.845.3301 Keywords: doped graphene, porphyrin, TD-DFT, electrical current, natural bond orbital

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1. INTRODUCTION

Among the several allotropes of carbon (nanotubes, fullerenes, diamond, graphite, graphene) the one that centers our attention during present times is graphene (G). It is a planar monolayer of carbon atoms which are arranged into a two-dimensional (2D) honeycomb lattice. Much of the interest in graphene and graphene-like materials is because their large surface area, high thermal conductivity, and sp2 hybridized orbitals.1 Graphene has unique electrical transport, mechanical,2-4 thermal5-7 and optical8 properties, which have found applications in various fields such as sensors, transparent electrodes, solar cells, energy storage devices, photodetectors, nanocomposites, etc.9 Graphene also has biological applications, for example, graphene-based sensors have been fabricated for detecting bacteria in the tooth enamel10 and glucose.11 There are several studies of the doping of graphene with transition metals,12-13 nitrogen,14-15 boron,16 phosphorus,17-18 sulfur,19 halogens,20 and hydrogen.21 Doping by replacement of a carbon atom by a dopant produces changes in the electronic density according to the type of dopant. Liu et al.22 have shown that it is possible to tune the graphene properties, thus facilitating the charge transfer between neighboring carbon atoms and therefore, increasing the usefulness of these materials. One of the first studies of dopant atoms in graphene was nitrogen (N) and nitrogen-based graphenes have been proposed as capacitors, showing that the better capacitance occurs for N-sites at basal planes than at edges.23 Graphene is also considered to be a good electrocatalyst for oxygen reduction reactions,24-25 and a good anode for high-power and high-energy lithium ion batteries specially under high-rates of charge and discharge conditions.26 2 ACS Paragon Plus Environment

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Theoretical and experimental studies have shown that the doping of graphene by boron (B) would produce a p-type semiconducting graphene that could enhance its electrical conductivity.27-29 It has been shown using Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) that multi-walled, single-walled and doublewalled carbon nanotubes as well as graphene can be doped with boron and nitrogen.30 But the dopant concentration could affect the conductivity of the material.31-32 The doping and control of conduction type, p or n, could be attained by co-doping of B and N in a BCN film.33-34 Phosphorus (PHP) doping of graphene has also been studied; it showed a band gap opening with respect to graphene (zero-gap semiconductor), and different methods to obtain these materials have been proposed, such as chemical functionalization and doping with heteroatoms. 35-37 Functionalization of graphene have attracted widespread attention because it combines the optical, chemical, magnetic and electric properties of both fragments and can feature new properties that could be useful for a variety of applications. Considering that porphyrins exhibit good optoelectronic properties, because they have an extended π-electron network (18 π) and present absorption in the visible light region, the synthesis of hybrid nanomaterials based in porphyrin and graphene has been encouraged.38-39 Mono amine phenyl substituted porphyrin was covalently functionalized with graphene oxide via an amide group in solvent dimethylformamide, and significantly improves the solubility and dispersion stability of the graphene-based material in organic solvents.40 GrapheneTetraphenylporphyrin (TPP) and graphene-PdTPP hybrid materials were also obtained and the covalent linkages confirmed by Raman and FTIR spectroscopy.41



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Taking into account the characteristics of graphene and porphyrin, the objective of this work is to study how the doping in graphene and in porphyrin affects the electronic properties at the ground and excited states of the complex, thus we can understand how at a molecular level the charge transport properties in graphene could be tuned for the development of a carbon-based nanoelectronics, nanosensing, nanocatalysts, among other applications.

2. COMPUTATIONAL DETAILS We analyze molecular complexes consisting of graphene (G), graphene doped with boron G(B), with nitrogen G(N), or with phosphorus G(PHP) bonded through an amide group to a porphyrin (P) or to a porphyrin with an aza nitrogen located in the meso position (Figure 1). A cluster formed by 19 hexagonal cells is used as a graphene model. Doped graphene is obtained by substituting a total of six carbon atoms (X) of the same row of rings. The porphyrins are monoamine-5,10,15,20-tetraphenylporphyrin (TPP) and the corresponding tetra-aza substituted in the four meso positions are the TPP(N). In our notation, we name the complexes G-P(N) and G(X)-P(N) where X = N, B, or PHP, in order to refer to the complexes with four aza nitrogen atoms in the porphyrin and to the corresponding no aza by G-P and G(X)-P where X = N, B, or PHP.


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H

H

H

H

H

H

H

H C2 X

X

X

H

C3

X

Y=C, N Y

X

X

O

C1

H

NH

N H

H

HN

Y

Y N

NH H

H H

H

Y

H

X=C, B, N, PHP

Figure 1. Molecular structure of graphene-porphyrin complexes. All the complexes are fully optimized with density functional theory (DFT) at the M06/631G(d,p) level of theory. M06 is a meta-hybrid generalized gradient approximated (GGA) functional with 27% of Hartree Fock electronic exchange. This functional is parameterized for main group, organometallics kinetics and non-covalent bonds.42 M06 performs very well the effect of dispersion forces. It uses an s6 scaling factor for the Grimme´s long range dispersion correction of 0.25. Owing to the charge transfer that graphene and porphyrin could experiment produced by their extended π networks, we used the functional M06. Harmonic vibrational frequency calculations are carried out for each complex in order to verify that the calculated structure correspond to a minimum energy structure. All of these structures correspond to the ground state of the complexes. The same procedure is applied to the fragments of graphene (doped and undoped) and porphyrin (aza N and no aza).



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Using the previously optimized structures, UV-vis electronic absorption spectra were calculated in the solution phase within the time dependent DFT methodology (TD-DFT) as implemented in the Gaussian 09 package.43 A total of 50 excited states for the fragments (graphene and porphyrin) and the corresponding complexes were calculated as vertical excitations. In the case of aza N complexes, we calculate up to 160 excited states in order to scan the spectra up to 300 nm. We chose the solvent dimethylformamide (DMF) (dielectric constant ε=37) for performing the theoretical calculations in solution phase according to the available experimental data of the complexes.40 The calculations including the solvent were performed with the self-consistent reaction field (SCRF) using the Conductor Polarizable Continuum Model (CPCM)44-47 and a molecular-shaped cavity built with the United Atom Topological Model48 applied on atomic radii of the Universal Force Field (UFF). All the calculations for the ground and excited states are performed with the Gaussian 09 package.43 The analysis of charges and electronic population is performed with the Natural Bond Orbital (NBO) scheme.49-50 In order to obtain current-voltage characteristics of graphene-porphyrin complexes, we implemented molecular junctions by attaching single gold atoms at both ends of the fully optimized structure of each molecule (see Figure 2). The gold atoms are further connected to gold nanoelectrodes represented in the formalism by their DOS. Single point energy calculations are performed by applying finite electric dipole field in the direction of the AuAu bond corresponding to each bias voltage using the Gaussian 09 package.43

The

Hamiltonian and overlap matrices are then extracted for each bias voltage and for each molecule. The DOS of the gold nanoelectrodes are obtained using the Crystal program.51 The DOS of the gold electrodes provide the source of electrons and the role of single gold


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atoms directly attached to the molecule is to produce an adequate geometry at the interface of molecule-electrode junction. The Hamiltonian and overlap matrices together with the DOS of nanoelectrodes are then utilized by our in-house GENIP52-54 program in order to obtain current-voltage curves. The GENIP program applies DFT and Green’s function procedures to calculate the current-voltage characteristics for a single molecular junction and has been successfully used for thioalkanes,53 graphene-based sensors,55-56 cobaltphthalocyanine,57-58 among others. The detailed formulation of the program can be found elsewhere.53

Figure 2. Boron doped graphene bonded to aza porphyrin attached to Au terminals. Carbon (gray), hydrogen (white), nitrogen (blue), oxygen (red), boron (pink) and gold (yellow).

3. RESULTS AND DISCUSSION 3.1 Molecular Geometry. The ground states of the fragments (graphene and porphyrin) and complexes are fully optimized and some relevant geometrical parameters are presented 7 ACS Paragon Plus Environment


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in Table 1. The optimized molecular geometry of the eight complexes is displayed in Figure 3.

G-P

G-P(N)

G(B)-P

G(B)-P(N)

G(N)-P

G(N)-P(N)

G(PHP)-P G(PHP)-P(N) Figure 3. Optimized molecular geometry of graphene-porphyrin complexes obtained at the M06/6-31G(d,p) level of theory. The dopant atoms are shown as balls. C (gray), B (pink), N (blue), and PHP (orange)

We found that the dopant atoms yield several effects on the molecular geometry: firstly we analyze the planarity of doped graphene and compare it with the undoped one. As illustration of the planarity, we examine the dihedral angle ∠C1XC2C3 (Figure 1). The


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dihedral is completely planar (0.0°) in G and G(N) (0.1°); however, presents a slightly planar deviation in G(B) (2.3°) and a strong planar deviation for G(PHP) (50.8°). The distortion of graphene takes place at the dopant atom sites and depends on the actual dopant. Boron has valence electrons that do not reach the octet; this affects the electronic distribution in the valence region producing a slight rise out the plane. Around phosphorus, its nonplanarity in graphene is singular. PHP is larger than B and N, and form a pyramidallike bonding configuration, where PHP rises from the graphene plane 1.8 Å. The bond length C1-P of 1.79 Å and angle ∠C1PC2 of 96° are comparable to the reported ones of 1.77 Å and 99° calculated at the PBE/DNP level of theory.59 When porphyrin is bonded to graphene (see Figure 3), we can notice that undoped graphene does not show a change with respect to the isolated one, for G(N)-P a slight (3.05°) increase of distortion in graphene similar to that obtained for G(B)-P (3.43°), and a larger distortion occurs for graphene in G(PHP)-P (47.1°) in relation to G(PHP). We observe that the structures change from an extended form in undoped graphene-porphyrin to a nearly perpendicular form for doped graphene-porphyrin. The non-planar doped graphene obtained for boron, nitrogen, and phosphorus dopants gives account of a non-homogeneous electronic density in doped graphene, allowing an approaching of the porphyrin toward graphene and shaping a perpendicular geometry between the fragments. For aza N doping in graphene-porphyrin complexes, we found that graphene keeps its planarity for undoped (0.1°) and doped with B (1.1°) and N (1.7°), but in the complex with phosphorus, the distortion of graphene remains (46.3°). The complexes are more extended and the reason could be attributed to the lone pairs of aza nitrogen atoms of the porphyrin, which, based in the NBO analysis, are of 100% π character. These lone pair would 9 ACS Paragon Plus Environment


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participate in the extensive conjugated π current of the macrocycle, leading to a more symmetric and planar structure and avoid that the porphyrin unit bends toward the doped graphene. Table 1 also indicates that the binding of graphene with porphyrin does not change the bond length C-C of undoped graphene of 1.4 Å suggesting that the sp2 hybridized orbitals are not altered. The B, N, or PHP doping in graphene makes the complexes C-X bond lengths smaller with respect to the isolated doped graphene, with the exception of G(B)-P, which is slightly larger; however, similar values are obtained when both, porphyrin and graphene, are doped. The results also indicate that the binding of doped graphene with porphyrin favors the formation of sp2 hybridized orbitals for graphene with the exception of phosphorus with a C-P distance near to 1.8 Å. Table 1. Relevant optimized geometrical parameters, distances (Å) and dihedral angles (deg), of the graphene-porphyrin based complexes calculated at the M06/6-31G(d,p) level of theory in the gas phase. Graphenes

dX-C1|

Complexes

∠C1XC2C3

dX-C1|

Complexes

∠C1XC2C3

dX-C1| ∠C1XC2C3

G

1.424|0.00

G-P

1.425|0.04

G-P(N)

1.425|0.13

G(B)

1.471|2.29

G(B)-P

1.481|3.43

G(B)-P(N)

1.465|1.14

G(N)

1.401|0.07

G(N)-P

1.394|3.05

G(N)-P(N)

1.395|1.67

G(PHP)

1.826|50.78

G(PHP)-P

1.809|47.14

G(PHP)-P(N)

1.760|46.30


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3.2 Electronic Absorption Spectra. UV-vis electronic absorption spectra for each of the eight complexes are obtained (Figure S1) and their analysis is focused in the doping effect on the electronic transitions. Such doping effects are due to the change of a C to a X atom (X = B, N, or PHP) in the graphene and, on other hand, to the change of the meso carbon to an aza nitrogen atom in the porphyrin. 3.2.1 Calibration of Density Functional. Previous studies performed by our group using TD-DFT calculations with a set of several functionals have showed that the simulation of the UV-vis electronic absorption spectra is a complex task and depends on the size and nature of the molecular system.60-64 Firstly, we performed a calibration procedure which consist of TD-DFT calculations of the electronic absorption spectra of porphyrin (P), graphene-porphyrin (G-P) and their aza derivatives P(N) and G-P(N), using the meta-GGA hybrid functional M06. This functional has a 27% Hartree Fock exchange. We also use the long-range corrected functional, CAMB3LYP,65which combines the hybrid qualities of B3LYP (including 20% Hartree Fock exchange) and the long-range correction of Tawada.66 We analyzed the theoretical results of Soret and Q bands corresponding to the porphyrin and compare it with the available experimental data. Table 2 shows the theoretical results obtained of the absorption bands (Soret and Q), including the wavelength, excitation energy, oscillator strength, electronic transition, number of the excited states and their assignments.



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Table 2. Electronic transitions of the absorption characteristic bands of porphyrin fragments P and P(N) as well as graphene-porphyrins complexes G-P, and G-P(N), including the number of the excited state (ES), wavelength (λ), excitation energy (E), oscillator strength (f), and their corresponding assignment of their electronic excitations in terms of their respective fragments calculated with M06(CAM-B3LYP)/6-31G(d,p) in condensed phase (solvent: N,N-dymethylformamide).

ES

Electronic

λ/nm (E/eV) f

Assignment

ES

Electronic

λ/nm (E/eV) f

Assignment

Transition

Transition P

P(N)

CAM-B3LYP 1

H→L+1

636 (1.95) 0.047

P→P (Q)

1

H→L

756 (1.64) 0.934

P→P (Q)

2

H→L

616 (2.01) 0.398

P→P (Q)

2

H-1→L

650 (1.91) 0.831

P→P (Q)

3

H-1→L

417 (2.97) 2.085

P→P (S)

4

H-3→L

350 (3.55) 0.003

P→P (S)

1

H→L

640 (1.94) 0.309

P→P (Q)

1

H→L

696 (1.78) 0.857

P→P (Q)

2

H→L+1

632 (1.96) 0.167

P→P (Q)

2

H-1→L

620 (2.00) 0.746

P→P (Q)

3

H-1->L

453 (2.73) 1.160

P→P (S)

12

H-5→L

355 (3.49) 0.048

P→P (S)

M06

G-P

G-P(N)

CAM-B3LYP 1

H-2→L

591 (2.10) 0.038

P→P (Q)

1

H→L

756 (1.64) 0.970

P→P (Q)

2

H-2→L+1

541 (2.29) 0.066

P→P (Q)

2

H-1→L

672 (1.85) 0.874

P→P (Q)

4

H→L+3

450 (2.75) 0.011

G→G

8

H-3→L+2

378 (3.28) 2.478

G→G

6

H-3→L

383 (3.24) 2.320

P→P (S)

11

H-4→L

354 (3.50) 0.002

P→P (S)

1

H-2→L

586 (2.12) 0.059

P→P (Q)

1

H→L

700 (1.77) 0.801

P→P (Q)

2

H-2→L+1

555 (2.23) 0.090

P→P (Q)

4

H-1→L

638 (1.94) 0.793

P→P (Q)

9

H-1→L+3

438 (2.83) 2.307

G→G

13

H-3→L+2

437 (2.84) 1.737

G→G

14

H-3→L

404 (3.07) 1.658

P→P (S)

22

H-4→L

383 (3.24) 0.002

P→P (S)

M06


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Overall, we find very similar results between M06 and CAM-B3LYP for the electronic transitions between occupied and virtual molecular orbitals, position of the absorption band, and the trends of oscillator strengths. In the case of the systems without aza nitrogen, P and G-P, the functional CAM-B3LYP predicts blue-shifted Soret and Q bands with respect to M06, with exception of the band at 591 nm (Q). For the systems with aza nitrogen, that is P(N) and G-P(N), the functional CAM-B3LYP predicts red-shifted Q bands and blue-shifted Soret bands with respect to M06. Interestingly, both functionals yield practically the same electronic transitions between the molecular orbitals, although M06 predicts more excited states than CAM-B3LYP in almost the same spectral region. For porphyrin, P, both functionals predict an intense Soret band at 453 nm (2.73 eV) for M06 and at 417 nm (2.97 eV) for CAM-B3LYP, which are compared with that observed experimentally at 419 nm (2.96 eV),40, 67 yielding deviations of 34 and 2 nm, respectively. The functionals also predict two weak Q bands that are compared to the experimental bands observed at 517, 556, 592, and 651 nm.67 The theoretical bands at 640-632 nm (M06) and 636-616 nm (CAM-B3LYP) compared well with the experimental ones at 651-592 nm. As reference, we can compare our results of P(N) with a porphyrazine which holds four aza nitrogen atoms in the meso position but it has not the phenyl and amine groups. Porphyrazine presents two intense bands, Soret at 330-350 nm and Q at 530-620 nm.68 On the other hand, a phthalocyanine that also has four aza N atoms located each one between two pyrrol rings but differs with porphyrin by a benzene ring bonded to each pyrrolic ring in the beta positions, features experimentally two Q bands located at 623 and 685 nm, and one Soret band at 340 nm, all of similar intensities.69 In our work we obtain two absorptions for Q band at 620 and 696 nm for M06 and at 650 and 756 nm for CAM-B3LYP, which are well compared with the Q bands for 13 ACS Paragon Plus Environment


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phthalocyanine, where the higher deviation to the experimental values is found for CAMB3LYP. In the case of Soret band, both functionals predict similar values and present a good agreement with the experimental ones for porphyrazine and phthalocyanine, 355 nm (M06) and 350 nm (CAM-B3LYP), respectively. For the G-P complex, we found that Soret band calculated with M06 (404 nm) features a better agreement with the observed value at 419 nm40, 67 than CAM-B3LYP (383 nm). The Q bands predicted for both functionals, 555-586 nm for M06 and 541-591 nm for CAMB3LYP, are localized in the spectral region 500-700 nm observed in experimentals.40, 67 The latter bands are observed at the same wavelengths than the porphyrin fragment, which are at 517, 556, 592 and 651 nm. Xu et al.40 showed that the UV-vis spectrum of tetraphenylporphyrin-NHCO-graphene in solution does not feature a significant change in the absorption bands corresponding to the porphyrin region when is compared to the spectrum of mono-aminetetraphenylporphyrin.40 For the G-P(N) complex, experimental data are not available and the theoretical results obtained for both functionals follow the same trend as P and P(N), in the sense that if N is included in the complex, the Q bands are red-shifted and Soret bands are blue-shifted. The aza nitrogen atoms are donor and contribute with electronic density toward the macrocycle, thus leading to absorptions of lower energy (longer wavelength). As conclusion of the calibration procedure, both functionals predict a similar behavior for the electronic spectra but M06 presents a slightly better agreement to the experimental data, hence we will continue the analysis of the electronic spectra of the remaining fragments and complexes with the functional M06. 3.2.2 Doping Effect on the Electronic Spectra. The results of the electronic spectra show two kind of charge transfer (CT) occurring in these complexes: (a) an intramolecular CT is 14 ACS Paragon Plus Environment

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shown for graphene (G→G) and for porphyrin (P→P) (Table 3); and (b) an intermolecular CT is found, giving account of electronic transitions such as P→G and G→P (Table 4). Firstly, we will discuss those electronic transitions more relevant for graphene (G→G) and for porphyrin (P→P). In the case of graphene (G) we obtain two absorptions located at 431 nm and 270 nm (4.59 eV), where the latter is well compared to the observed bands at 269 nm (4.61 eV) by Zhang et al.70 or at 270 nm by Li et al.,71 both corresponds to a reduced graphene oxide and to a π→ π* electronic transition that suggests the restoration of C=C bonds in the graphene sheet by the removal of oxygen-containing groups. The more intense theoretical band at 431 nm is associated to a HOMO→LUMO+1 electronic transition where the electronic density is located at the center and periphery, in contrast to the less intense band at 270 nm corresponding to a HOMO-3→LUMO+3 electronic transition located at the graphene center. Nitrogen-doped graphene (G(N)) has been proposed as a visible-light photocatalyst for the hydrogen generation from water-methanol mixtures, where samples of G(N) have shown a good response to the hydrogen evolution during a period of time when they are irradiated at 355 nm (3.49 eV) and 532 nm (2.33 eV).72 The observed bands compare well with the theoretical absorptions at 385 nm (3.22 eV) and 501 nm (2.48 eV), respectively. To our knowledge, UV-vis absorption spectra of G(B) and G(PHP) are not available. In overall, we found that all the porphyrin-graphene complexes without aza nitrogen atoms feature the characteristic bands of the porphyrin: one strong band (Soret) and two weak bands (Q). We noted that in complexes with aza N, the Q bands are of larger intensity than the Soret band, following a similar behavior than porphyrazines and phthalocyanines.



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Taking as reference the porphyrin absorptions, graphene binding to porphyrin leading to G(X)-P produces in all the cases (four complexes) a blue-shift in the absorptions of porphyrin, Soret and Q bands, indicating that the electronic transitions are of higher energy. Depending of the complex, the Q and Soret bands are blue-shifted in about 40-80 nm when doped or undoped graphene is bonded to porphyrin; however, all the absorptions occur in the visible range. The absorption window of G(X)-P goes from 402 to 589 nm which is compared to the one for porphyrin (P), which goes from 453 to 640 nm, and both belong to the visible-light region. Considering the absorptions of P(N), the G(X)-P(N) complexes present red-shifted Q bands, with exception of boron complexes, and red-shifted Soret bands for all complexes, with an absorption window of 327 to 700 nm for the complexes in comparison with 355 to 696 nm for P(N). We note that the binding between graphene and N-aza porphyrin maintains the absorption in the visible light range demonstrating that porphyrin does not lose their optical properties. The doping on the graphene for G(X)-P complexes produces small changes in the absorption of the Q (≈ 3 nm) and Soret (≈ 7 nm) bands in comparison with G-P. We noted that boron doping leads to a blue-shift in the absorptions, however nitrogen and phosphorus doping result in a red-shift of the absorptions. In relation to the intensities of both bands, there is no regular trend in the complexes when the graphene is doped. The doping on the graphene for G(X)-P(N) complexes yields small changes in the Q absorption band (≈5 nm) in comparison to G-P(N), with exception of Q bands for G(B)-P(N) that are blue-shifted up to 86 nm. Soret bands are blue-shifted in about 50 nm with respect to G-P(N) that shows the doping effect on graphene.


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It is interesting to note that, except for the undoped graphene complexes (G-P, G-P(N)), the maximum of the spectra for G(X)-P and G(X)-P(N) corresponds to porphyrin-porphyrin electronic transitions associated to the Soret bands in the case of G(X)-P and to the Q bands in the case of G(X)-P(N). This reflects the porphyrin-like character of G(X)-P complexes and the phthalocyanine-like character of G(X)-P(N). The properties of the porphyrin as photosensitizers are favored when the absorptions are red-shifted because they are lower in energy and could have more applications as sensors, cancer imaging and therapeutic procedures. The electronic spectra of graphene-porphyrin also show G→G transitions that are lowest in intensity than P→P, except for the undoped graphene (G-P, G-P(N)). Table 2 shows that the G→G transitions of higher intensities occur at 438 and 437 nm for G-P and G-P(N), respectively, which are compared with 431 nm in G. In all the cases the doping on graphene by B, N and PHP yields larger wavelengths than that observed for G (431 nm). The doping and no doping in graphene produces a red-shift of the absorption G→G indicating that the intramolecular charge transfer in graphene is favored for the graphene-porphyrin complexes.



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Table 3. Electronic transitions of the characteristic bands of graphene (G) and porphyrin (P) including the number of the excited state (ES), wavelength (λ), energy (E), oscillator strength (f) and the corresponding assignment of the electronic excitation in terms of the respective fragments calculated with M06/6-31G(d,p) at the condensed phase (solvent: N,N-dymethylformamide). ES

Electronic

λ/nm (E/eV) f

Assignment

ES

Electronic

Transition

λ/nm (E/eV) f

Assignment

Transition

G

G(B)

3

H→L+1

431 (2.88) 1.599

G→G

25

H-8→L

529 (2.35) 0.603

G→G

39

H-3→L+3

270 (4.59) 0.633

G→G

37

H-12→L

414 (3.00) 0.441

G→G

G(N)

G(PHP)

20

H-3→L

501 (2.48) 0.635

G→G

12

H→L+5

552 (2.25) 0.213

G→G

29

H-4→L+1

385 (3.22) 0.350

G→G

21

H-1→L+5

448 (2.77) 0.138

G→G

G(B)-P

G(B)-P(N)

32

H-2→L+3

584 (2.12) 0.037

P→P (Q)

30

H-2→L+3

614 (2.02) 0.747

P→P (Q)

33

H-2→L+4

554 (2.24) 0.037

P→P (Q)

33

H-9→L+2

578 (2.14) 0.122

G→G

35

H→L+5

536 (2.31) 0.217

G→G

34

H-11→L

572 (2.17) 0.576

P→P (Q)

80

H-3→L+3

402 (3.08) 0.737

P→P (S)

137

H-27→L

331 (3.75) 0.393

P→P (S)

G(N)-P

G(N)-P(N)

22

H-3→L+1

589 (2.10) 0.063

P→P (Q)

17

H-2→L+1

695 (1.78) 0.790

P→P (Q)

24

H-3→L+2

558 (2.22) 0.085

P→P (Q)

20

H-4→L+1

635 (1.95) 0.885

P→P (Q)

27

H-2→L+7

514 (2.41) 0.106

G→G

25

H-3→L+4

559 (2.22) 0.170

G→G

42

H-5→L+2

406 (3.06) 1.479

P→P (S)

100

H-4→L+12

330 (3.76) 0.128

P→P (S)

G(PHP)-P

G(PHP)-P(N)

21

H-3→L+2

589 (2.10) 0.066

P→P (Q)

13

H→L+1

696 (1.78) 0.767

P→P (Q)

22

H-6→L

571 (2.17) 0.292

G→G

18

H-2→L+1

635 (1.95) 0.797

P→P (Q)

25

H-3→L+3

556 (2.23) 0.026

P→P (Q)

20

H-1→L+5

596 (2.08) 0.019

G→G

47

H-4→L+3

411 (3.02) 1.640

P→P (S)

112

H-2→L+16

327 (3.79) 0.110

P→P (S) 18

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We have also found a set of electronic transitions involving an intermolecular charge transfer from graphene to porphyrin and vice versa. Table 4 shows as illustration the two lower energy bands G→P and P→G of the complexes calculated with M06 in terms of the molecular orbital surfaces. We found that, excepting for G(N)-P and G(N)-P(N), the complexes doped with aza N in porphyrin present a larger excitation wavelength for both Soret and Q bands. Thus, it is confirmed that the aza N effect produces a red-shift of the absorption bands. It is important to mention that CAM-B3LYP calculations carried out for G-P and G-P(N) also show this kind of charge transfer between the fragments. Considering the longer excitation wavelength, we obtained for G-P a P→G transition at 388 nm (3.19 eV) and a G→P transition at 322 nm (3.85 eV). In the case of G-P(N), we found that the absorption for the P→G and G→P transitions occur at 443 nm (2.80 eV) and at 344 nm (3.61 eV), respectively. Both functionals give account of a charge transfer between graphene and porphyrin in the excited states. At the experimental level, it was observed for porphyrin-graphene complexes a quenching of fluorescence (or phosphorescence) and reduced lifetimes suggesting an excited state energy/electron transfer between graphene and the covalently attached porphyrin molecules, as we obtained theoretically (Table 4).41 On the other hand, Pham et al.73 investigated the interaction between nitrogen-doped graphene and tetraphenylporphyrin from high resolution scanning tunneling microscopy (STM) imaging and spectroscopy. The authors found evidence of a charge transfer at the doping sites from the graphene to the porphyrins due to the donor character of the nitrogen atoms. They also found that the porphyrin as single molecule, cluster and linear chain is



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distributed in nitrogen-doped graphene in an interaction radius of 8 Å around the nitrogen atoms. This indicates that the interaction of graphene with porphyrin results in a charge density redistribution around the nitrogen atoms in graphene rather than a chemical interaction with nitrogen.73 Table 4. Electronic transitions including the wavelength (nm), excitation energy (eV), the oscillator strength (f), and the molecular orbital surfaces of the two lower energy bands G→P and P→G calculated with M06/6-31G(d,p) at the condensed phase (solvent: N,Ndymethylformamide).

Electronic Transition G-P

Electronic Transition G-P(N)

G→P

P→G

H→L

H→L+1

464 nm

690 nm

(2.67 eV)

(1.80 eV)

f=0.028

f=0.107 P→G

G→P

H-2→L+2

H-2→L

429 nm

466 nm

(2.89 eV)

(2.66 eV)

f=0.046

f=0.003

G(B)-P

G(B)-P(N) P→G

P→G

H-2→L

H→L+1

1444 nm

3700 nm

(0.86 eV)

(0.34 eV)

f=0.019

f=0.003 G→P

G→P


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H→L+3

H-6→L

614 nm

948 nm

(2.02 eV)

(1.31 eV)

f=0.015

f=0.002

G(N)-P

G(N)-P(N) G→P

G→P

H-2→L+1

H-3→L+1

816 nm

750 nm

(1.52 eV)

(1.65 eV)

f=0.015

f=0.001 P→G

P→G

H-5→L

H-6→L

452 nm

383 nm

(2.74 eV)

(3.24 eV)

f=0.003

f=0.002

G(PHP)-P

G(PHP)-P(N) G→P

G→P

H-1→L+2

H-1→L+1

594 nm

709 nm

(2.09 eV)

(1.75 eV)

f=0.022

f=0.046 P→G

P→G

H-4→L+1

H→L+3

555 nm

690 nm

(2.23 eV)

(1.80 eV)

f=0.011

f=0.008

In summary, the study of the electronic absorption spectra shows that: (a) porphyrin is a good photosensitizer when covalently interacts with doped or undoped graphene because it 21 ACS Paragon Plus Environment


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absorbs in the visible light region; (b) the replacement of meso carbon by aza nitrogen atom in porphyrin produces a red-shift in the Q bands and (c) charge transfer between graphene and porphyrin occur in the excited states with energies belonging to the visible light region.

3.3 Electronic Properties 3.3.1 HOMO-LUMO Gap. We have also studied the doping effect in graphene and porphyrin on the HOMO-LUMO Gap (HLG). This gap provides insight on the chemical stability. It is known that graphene is a semiconductor but it has a zero band gap for an infinite size graphene. It has been shown that the HOMO–LUMO gap decreases as res the size of the graphene increases, an example of this by Kheirabadi et al.,74 the HLG calculated for a set of graphene clusters decreased logarithmically as function of the carbon number and a graphene structure consisting of 500 carbon atoms is the threshold of infinite size for zero gap energy. Although the structure reported in our work for undoped graphene has not a zero HLG (3.12 eV), and in order to determine the doping effect on graphene and on porphyrin, we will compare the doped complexes studied in this work with the undoped graphene calculated here. The finite cluster structure that we used in this work is composed of 58 carbon atoms and 18 hydrogen atoms ordered in 19 hexagonal rings. The HLG for the cluster C58H18 calculated at the M06/6-31G(d,p) level of theory is 3.12 eV and compares well with a B3LYP/6-31G(d,p) calculation reported for C54H18 (cluster with vacancies) with a gap value of 2.82 eV.74 Table 5 display the HLG values calculated for the graphene and porphyrin complexes.


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Table 5. HOMO-LUMO gap (HLG) (eV) of the graphene-porphyrin complexes calculated in vacuum at the M06/6-31G(d,p) level of theory.

Graphenes

HLG

Complexes

HLG

Complexes

HLG

G

3.12

G-P

2.94

G-P(N)

1.66

G(B)

0.55

G(B)-P

0.65

G(B)-P(N)

0.21

G(N)

0.71

G(N)-P

0.91

G(N)-P(N)

0.90

G(PHP)

0.98

G(PHP)-P

0.94

G(PHP)-P(N)

0.23

The doping atoms on graphene leading to G(B), G(N) and G(PHP) produce an important decrease in HLG indicating that the doped molecular structures are more reactive to a change in the electronic distribution than undoped graphene. This behavior also suggests that an electronic transport from the occupied to unoccupied molecular orbitals is favored. In relation to the graphene-porphyrin complexes, G-P, the doping on graphene to G(B)-P, G(N)-P), and G(PHP)-P, decreases dramatically the HLG from 2.94 to 0.65, 0.91 and 0.94 eV, respectively. This is 2 eV reductions for nitrogen and phosphorus dopants and 2.3 eV for boron. It is important to note that doped graphene (B or PHP) has similar values of HLG when they are bonded to porphyrin, but the advantage to use the latter is its range within the visible region absorption. From G(X)-P complexes, when the porphyrin is doped with aza nitrogen atoms, the HLG decreases 1.28 eV for undoped graphene G-P(N) and 0.44, 0.01, and 0.71 eV for boron, nitrogen and phosphorus dopants, respectively. It means that the doping on the graphene component has a larger effect on the HLG than the doping on the porphyrin one. In summary the results predict: (a) in comparison with graphene (3.12 eV), the dopant atoms (B, N, and PHP) could favor the electron transfer from the occupied (HOMO) to unoccupied (LUMO) molecular orbitals for both doped graphene and doped graphene23 ACS Paragon Plus Environment


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porphyrin owing to their shorter HLG and in consequence a lower energy; (b) the aza nitrogen dopant atoms on porphyrin produces a decrease of the HLG that is more important than the decrease in the corresponding complexes without the aza N. This behavior only happens for boron and phosphorus dopant atoms because the complexes with nitrogen dopant atoms do not change; (c) the results indicate that the doping in both fragments modify the chemical stability because the HLG reduces. On the other hand, we also investigated the effect on the HOMO-LUMO gap of the structural changes in graphene such as: (a) the size of graphene, (b) number and (c) position of the dopant atoms in graphene. We optimized a G(N)-P(N) complex in vacuum with one nitrogen-dopant atom having 19 and 30 hexagonal rings (Figure S2) at the same level of theory than the other complexes on Table 5. We found that increasing the size of graphene (30 rings) decreases the HLG by 1.14 eV with respect to the 19 rings HLG of 1.49 eV, similar to that reported by Kheirabadi et al.74 Note that the graphene clusters have 2.300 and 1.810 basis functions, respectively, which demand a high computational cost, thus the cluster of 19 rings is a good tradeoff for this kind of studies. Considering the G(N)-P(N) complex with 1N and 19 rings, we optimized two structures at the same level of theory in vacuum for two different positions in graphene of the pyridinic type, at the edges of graphene and away from the edges (Figure S3 (a) and (b)). We obtained very similar HLG, 1.49 eV (at the edges) and 1.53 eV (away from the edges) indicating that the position of the dopant atom does not affect much the HLG. We also explore how the distribution of dopant atoms along the graphene affects the HLG energy. We optimize at the same level of theory the G(N)-P(N) complex with the 6N randomly distributed and then compare with the complex having the 6N distributed in a 24 ACS Paragon Plus Environment

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row (ordered structure) (Figure S3 (c) and (d)). We found for 6N randomly a HLG value of 1.09 eV and for the 6N in a row a value of 0.90 eV showing that a more ordered structure yields a slight decrease of the gap favoring excitations of lower energy. Finally, we also examine the effect of the number of dopant atoms on the HLG value. We compared the results for 1N (1.49/1.53 eV) with 6N (1.09/0.90 eV). We found a decrease in the HLG when more nitrogen-dopant atoms are included in graphene indicating that the reactivity of the complex is favored. 3.3.2 Natural Bond Orbital Scheme. Until here we have shown the dopant effect in both graphene and porphyrin on the HOMO-LUMO gap, but what is the reason for the decreasing trend? One can find some answers in the analysis of the atomic charges. We used the charges obtained from the natural bond orbital (NBO) scheme. Figure 4 shows as illustration the NBO charges of C and X (B,N,PHP) atoms belonging to one fragment (the same ring) of graphene of the G(X)-P and G(X)-P(N) complexes.

C7 C2 C3 C1 C4 C6 C5

B

N

P

G-P/

G(B)-P/

G(N)-P/

G(PHP)-P/

G-P(N)

G(B)-P(N)

G(N)-P(N)

G(PHP)-P(N)

C1

0.00/0.00

-0.73/-0.68

0.42/0.41

-0.81/-0.82

C2/B/N/P

0.00/0.00

0.67/0.64

-0.38/-0.37

1.13/1.01

C3

0.00/0.00

-0.55/-0.52

0.46/0.46

-0.81/-0.81

C4

0.00/0.00

0.02/0.01

-0.05/-0.05

0.01/0.01

C5

0.00/0.00

0.00/0.01

0.01/0.02

-0.03/-0.06

C6

0.00/0.00

0.05/0.03

-0.07/-0.07

0.01/0.02

Atoms



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

C7

0.00/0.00

-0.11/-0.09

0.07/0.07

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-0.34/-0.29

Figure 4. NBO charges (e) in vaccum of a graphene fragment of G(X)-P and G(X)-P(N) complexes calculated at the M06/6-31G(d,p) level of theory. Rings belong to the G-P and G(X)-P complexes and similar colors are obtained for complexes with aza N. Color code: green-positive charge, red-negative charge, black-zero charge.

The results indicate that in the undoped graphene-porphyrin (G-P and G-P(N)) the charge is zero giving account of an homogenous charge distribution along the graphene. Charge values are similar in the remaining rings of graphene, at exception of the carbon atoms located in the periphery of the graphene (-0.2) because the cluster is ended with hydrogen atoms in order to get a planar structure. Boron and phosphorus atoms yield negative charge values for those carbon atoms (C1, C3, C7) directly bonded to the dopant atom and C5 in the case of phosphorus. The other carbon atoms (C4, C5, C6) present near to zero charge values. In opposition to that, the doping with nitrogen atoms produces positive charge values for (C1, C3, C7). The results confirm that B and P atoms are p-type (provide positive carriers) dopants because generate sites with charge depletion. In contrast to that, N atoms in graphene would be n-type (provide negative carriers) dopants because yield sites with charge enhancement. Similar trend is confirmed by other reports where using the PBE functional yielded a charge for the nitrogen atom of -0.26 in N-doped graphene and a charge for the boron atom of 0.61 for Bdoped graphene.75 In the case of the phosphorus-doped complexes, we obtained similar values to NBO charges for the phosphorus atoms using the electronic population given by the Mulliken and Hirshfeld schemes. As an example, for the phosphorus atom shown in Figure 4, the Mulliken and Hirshfeld charges calculated for G(PHP)-P/G(PHP)-P(N) are 0.46/0.50 and 0.19/0.19, respectively. 26 ACS Paragon Plus Environment

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The dopant atoms, considered as perturbation to the charge density, modify the chemical stability of the complex, reflected in a shortening of HLG, and the conjugation of π electrons viewed by the hybridization of these atoms. Furthermore, the dopant atoms yield charged sites, positive or negative, along the graphene that are considered as potential reactive sites. The natural bond orbital scheme give us the hybridization of carbon atom C2 in undoped graphene-porphyrin (G-P) as (33% s, 67% p) that corresponds to a hybrid orbital sp2. For the doping on graphene in G(X)-P complexes, the boron atom has a character of (35% s, 65% p) and the nitrogen atom of (33% s, 67% p), indicating that in both cases the heteroatom has an hybridization of sp2. In the case of the phosphorus atom the NBO gives a character of (17% s, 83% p) that is nearly a sp3 which simply means the ability of creating a

π-character hole (or P-type semiconductor) in a 3-coordinated structure as opposed to the σ-character electron (or N-type semiconductor) created in a standard 4-coordinated semiconductor material such as Si or Ge. For the G(X)-P(N) complexes the character of the heteroatom gives for B (35% s, 65% p), for N (33% s, 67% p) and for PHP (18% s, 81% p) indicating that in these complexes the hybridization for B and N dopants is sp2 and for PHP is sp3.

3.4 Current Voltage Analysis. The current-voltage curves of undoped and doped graphene-porphyrin complexes are shown in Figure 5. For a positive bias at graphene end, the electrical conductance increases in the order G(N)-P > G(B)-P > G(PHP)-P > G-P. The trend can be explained by looking at the HOMO-LUMO gap (HLG) and the direction of bias voltage. The HLG is maximum (2.94 eV) in the case of undoped graphene-porphyrin



complex and minimum in the case of G(B)-P complex (0.65 eV). On the other hand, B and PHP atoms are p-type (provide positive carriers) dopants while N atoms in graphene are ntype (provide negative carriers) dopants as already discussed in previous section. Thus when the graphene-end is at higher potential, we obtain larger current in the case of G(N)-P complex followed by G(B)-P complex even though G(B)-P has a smaller HLG. As the bias voltage is reversed, we obtain much higher current in case of G(B)-P complex when compared to undoped, nitrogen and phosphorus doped graphene-porphyrin complexes. The current values for G(PHP)-P also increases on reversing the polarity of external voltage as in the case of G(B)-P, however the current values are smaller due to larger HLG. Basically, we found an interesting group of molecular diodes.76-78

Current (nA)

0.06 0.04 0.02 -3

-2

0 -1 0 1 -0.02 Voltage (V)

2

3

2

3

(a) 10 Current (nA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-3

0 -2

-1

-10

0

1

-20 -30 -40 Voltage (V)

(b)


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Current (nA)

30 20 10 0 -3

-2

-1

0 1 -10 Voltage (V)

2

3

(c) 0.2 Current (nA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-3

-2

-1-0.2 0 -0.6

1

2

3

-1 -1.4 -1.8 Voltage (V)

(d) Figure 5. Current-voltage curves for (a) G-P, (b) G(B)-P, (c) G(N)-P, and (d) G(PHP)-P graphene-porphyrin complexes. In all cases, positive current indicates current flowing from graphene (G) to porphyrin (P) i.e. left to right. All systems show properties of a diode device.

Figure 6 shows the theoretical current-voltage curves for G-P(N), G(B)-P(N), G(N)-P(N), and G(PHP)-P(N) complexes. Similar to the previous set of complexes, the current is maximum in case of G(N)-P(aza) when the graphene end is at higher potential. The trend in the electrical conductance follows the order G(N)-P(N) > G(B)-P(N) > G-P(N) > G(PHP)P(N). The HLG of the four complexes are in the order G(B)-P(N) > G(PHP)-P(N) > G(N)P(N) > G-P(N). When porphyrin is at a higher potential, the current is maximum in case of G(PHP)-P(N) due to smaller HLG and P being a positive dopant. The boron-doped G(B)P(N) complex shows a negative differential resistance, the reason for which needs to be



further investigated. Thus we observe that dopants can significantly affect the electrical conductance of graphene-porphyrin complexes. 0.1 Current (nA)

0.08 0.06 0.04 0.02 -3

-2

0 -1 0 1 -0.02 Voltage (V)

2

3

2

3

(a)

4 2 0 -3

-2

-1

0

1

2

3

-2 -4 Voltage (V)

(b) 120 80

Current (nA)

Current (nA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40 0 -3

-2

-1

0 1 -40 Voltage (V)

(c)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 Current (nA)

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0 -3

-2

-1

-1

0

1

2

3

-2 -3 -4 Voltage (V)

(d) Figure 6. Current-voltage curves for (a) G-P(N), (b) G(B)-P(N), (c) G(N)-P(N), and (d) G(PHP)-P(N) graphene-porphyrin complexes. In all cases, positive current indicates current flowing from graphene to porphyrin (left to right). Except for G(B)-P(N) all the other systems show a diode behavior.

The I-V curves for our complexes are very important because we want to understand if these systems perform charge transport between their components; how conducting or insulating they are; how variable or sensitive to foreign moieties they are such that they can act as sensors of molecules. Even more, at their excited states, these systems are of strong importance for solar cells as we know already that porphyrin absorbs in the visible light region and can transfer the excited electron toward a graphene or nanotube, thus the hole in porphyrin is paired with one electron from an electrolyte because our results indicated that there is a charge transfer between the fragments in the excited state. The theoretical importance of the I-V procedure used in this work is because it is an explicit indicator of electron transport that includes several effects such a: of the external fields; of the molecular orbital energies; of the nature localized-delocalized of the electrons in the frontier molecular orbitals; and of the electronic structure of the electron collectors. In addition, we also obtained current-voltage curves for G(N)-P complexes by doping graphene with single nitrogen atom at two different locations as well as doping at random 31 ACS Paragon Plus Environment


locations with multiple nitrogen atoms (Figure 7). Our results indicate that the change in the conductance depends both on number as well as location of nitrogen atoms. Current values for single nitrogen doped G(N)-P are one magnitude higher than those obtained for undoped G-P complex (Figure 5(a)). Moreover, we obtained lower currents, in the voltage range 0–2 V, when a nitrogen atom is located away from the edges (Figure 7a) as compared to the case when the nitrogen atom is at the edge of graphene (Figure 7b). Note that these results are coherent with the HLG values of 1.53 eV for complex Figure 7a and 1.49 eV for complex Figure 7b mentioned in section 3.3.1. However, at higher voltages (> 2V), we observe a reversal in behavior. On the other hand, if multiple nitrogen atoms are doped on graphene at random locations (Figure 7c), the currents are smaller than those obtained when nitrogen atoms are doped in a particular order (Figures 6c). These results are consistent with the HLG values obtained for the complexes, 1.09 and 0.90 eV for random and ordered structures, respectively. When compared to undoped G-P complex, the currents are lower for positive bias voltage and a larger current at negative bias voltage in case of randomly doped G(N)-P complex. In summary, we have obtained a wide range of possibilities to tune the electrical properties of these complexes. 1.6 1.2 current (nA)

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0.8 0.4 0 -3

-2

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0 1 voltage (V)

2

3

(a)


0.5 0.4 current (nA)

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0.3 0.2 0.1 0 -4

-2

0 voltage (V)

2

4

(b) 0.04 0 current (nA)

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-3

-1

1

3

-0.04 -0.08 -0.12 voltage (V)

(c) Figure 7. Current-voltage curves for (a, b) single nitrogen doped G(N)-P(N) complexes, and (c) multiple nitrogen atoms randomly doped G(N)-P(N) complex. In all cases, positive current indicates current flowing from graphene to porphyrin (left to right).

On the other hand, the charge transport has a strong dependence on the binding sites of the gold atoms. That is something unavoidable at atomistic scales. This is the reason why most calculations are performed with an identical setting of gold atoms and associated density of states (DOS) of the nanozised electrodes so all samples can be equally compared. This binding site may affect strongly the conductance of the junction depending on whether the molecular frontier molecular orbitals of the molecule and gold atoms localize or delocalize when affected by the continuum DOS of the nanoelectrodes. For the sake of completeness, 33 ACS Paragon Plus Environment


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the other strong factors affecting the conductance of the molecular junction are the applied external voltage and fermi energy of the two electrodes.

4. CONCLUSIONS Our goal is to find out nanosized systems that might have useful properties related to charge transport. The properties at the nanoscales are strongly dependent of the size of the sample that might be seen as a disadvantage but in most cases it is really a great advantage to be able to tune the properties of nanomaterials. In the specific case of the conductivity for instance, it would be almost impossible to change the conductivity of a macroscopic material by changing its size; however, this is entirely possible at the nanoscale. Considering our goal in this work, we used DFT and DFT-TD at the M06/6-31G(d,p) level of theory to explore the doping effects of B, N, and PHP on graphene and of aza N on porphyrin when graphene and porphyrin are bonded by an amide group leading to GP complexes. Structural, optical, electronic and transport properties are analyzed for the eight GP complexes. We found that porphyrins maintain their optical properties, Q and Soret bands, in the complexes, and with both dopings, in graphene and in porphyrin, the Q absorptions are red-shifted, which favors the P→P intramolecular electronic transitions. We also observe P→G and G→P intermolecular electronic transitions in the visible light region but are of lower intensity than P→P, giving account of the charge transfer between the graphene and porphyrin fragments. In relation to graphene, both dopings, by B, N, and PHP as well as by aza N, reduce the HOMO-LUMO gap indicating that the intra and intermolecular electronic transitions are favored in graphene-porphyrin complexes.


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The HOMO-LUMO gap and the nature of dopants (B, N, and PHP) plays an important role in determining the electrical conductance of graphene-porphyrin complexes. The HLG is the smallest in the case of B-doped graphene-porphyrin complexes. Boron and phosphorus are positive dopants while nitrogen is a negative dopant. The conductance of graphene increases 1-2 orders of magnitude when dopants are present and it is a maximum for the case of nitrogen-doped graphene-porphyrin complexes. We also found that phosphorus behaves as P-type dopant in the three-coordinated (graphene) systems as opposed to be a Ntype dopant in four coordinated systems (Si, Ge) and in most cases, the analyzed structures show a diode behavior. ASSOCIATE CONTENT Supporting Information. Electronic absorption spectra calculated for all fragments (porphyrin, graphene) and graphene-porphyrin complexes using the TD-DFT methodology; and optimized molecular structure models that differ in the size of graphene, position and number of dopant atoms in graphene. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected]; Phone: 56-2-27181137 * E-mail: [email protected]; Phone: 1-979-845-3301 Notes The authors declare no competing financial interests. ACKNOWLEDGEMENTS



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The authors thank the financial support from CONICYT-CHILE by Project FONDECYT N°1131002. JMS would like to acknowledge high-performance computing support provided by the Texas A&M Supercomputer Facility and the Texas Advanced Computing Center (TACC), as well as the financial support from the Lannater and Herb Fox Professorship.

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Doping Effects in the Charge Transport of GrapheneâPorphyrins

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