Perspective pubs.acs.org/JPCL
Exploring Single Molecules by Scanning Probe Microscopy: Porphyrin and Phthalocyanine Tianchao Niu*,†,‡ and Ang Li† †
State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, 865 Changning Road, Shanghai 200050, People’s Republic of China ‡ Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543 Singapore ABSTRACT: Utilizing single molecules as the building blocks for electronic devices is one promising pathway for microelectronic miniaturization. Exploring the molecular conformation and controlling the inter- and intramolecular bonding as well as their charge and spin configuration is essential for developing such novel devices. Scanning probe microscopy (SPM) is among the most powerful tools for characterizing and manipulating the single molecules at the atomic level. In this Perspective, a brief review of recent scanning probe microscopic studies of phthalocyanine, an excellent candidate for molecular electronic devices, is given. It is shown how phthalocyanine can be functionalized as a single-molecule switch by the state-of-art STM manipulation. The power of scanning probe techniques is demonstrated via these examples, and future challenges are pointed out.
“on” and “off” at the molecular scale. The properties of a single molecule sitting on top of the electrode are thus of great interest and crucial for the functioning. Accordingly, this Perspective is organized as follows. First, different types of phthalocyanine- and porphyrin-based molecular switches are introduced. The second part describes how the spin state of phthalocyanine molecules can be imaged and switched with STM. In the third section, surface reactions like metalation/ demetalation of phthalocyanine molecules are shown to be activated with atomic precision by applying a pulse voltage across the STM tip and sample. Last, we demonstrate the charge distribution within a single molecule, which can be distinctly mapped out with Kelvin probe force microscopy (KPFM). These works reveal different aspects of single molecules on the supporting substrate and their prospective application in electronic devices.
M
olecular electronics is an interdisciplinary area that has attracted great interest due to its prospect to extend Moore’s Law beyond the scale limits of conventional siliconbased integrated circuits.1 The drive to miniaturization and cost reduction of molecular devices has pushed the field to achieve the atomic-precise control of the molecule/electrode interfaces.2 Thus, exploring the molecular conformation,3 intra/intermolecular bonds,4,5 charge distribution,6 surface reaction,7−9 and molecular trapping10 at the single-molecule level would be essential for incorporating the most from the molecular specificity into the electronic devices.11−13 Specially, the size reduction relies on the molecular- or atomic-level control of fabrication and characterization; hence, innovative tools and methods are required.14,15 Among them, scanning tunneling microscopy (STM),16 atomic force microscopy (AFM),17 and many other scanning probe techniques18 feature ultrahigh spatial resolution with the capability of atomic/molecular manipulation that makes a tremendous impact on single-molecule electronic device research.19 Porphyrins and phthalocyanines represent an important class of functional organic semiconductors that have been widely used in molecular electronics.20 They offer an ideal system to demonstrate both fundamental physics, such as negative differential resistance,21 magnetoresistance,22 and the Kondo effect,23,24 and potential functionalities like molecular rotors25 and switches.26,27 In this Perspective, we focus on the recent progress in scanning probe microscopy (SPM) studies of porphyrin and phthalocyanine molecules in the past 3 years. In view of the motivation and grand challenge in this critical field, particular attention is given to the interfacial properties between the molecule and electrode. The molecular switch is recognized as one of the basic functional components in molecular electronics that can be turned © 2013 American Chemical Society
Porphyrin and phthalocyanine molecules are ideal candidates for both technical applications and fundamental research due to their versatile functionalities. Molecular Switch. The molecular switch operates in a way that the molecule is reversibly tuned between bistable or multistable states.28 In the case of phthalocyanine, the central coordinated Received: September 26, 2013 Accepted: November 14, 2013 Published: November 14, 2013 4095
dx.doi.org/10.1021/jz402080f | J. Phys. Chem. Lett. 2013, 4, 4095−4102
The Journal of Physical Chemistry Letters
Perspective
Figure 1. (a) Top view and side view of the ClAlPc and (b) single-molecule switch at 77K. The Cl-up ClAlPc (0) can be switched to Cl-down (1) by positive voltage pulses; (c) the corresponding lateral line profiles demonstrate the topographical differences between the Cl-up and Cl-down ClAlPc molecules. (d) Simulated STM image of the TbPc2 molecule by DFT calculation, showing the position of the eight lobes, and the side view of the TbPc2 molecule after structural optimization by using DFT calculations, showing the sandwiched structure. (e) Chirality manipulation of TbPc2 on Ir(111). STM topographic images of a TbPc2 molecule switched from the chiral state (a) to the achiral state (b) and further switched back (c), corresponding structural models of TbPc2 with different chirality properties, are shown below. (d)32 First-principle calculations were performed by using the VASP code, employing a plane wave basis set and projector-augmented wave (PAW) potentials to describe the behavior of valence electrons. A generalized gradient Perdew−Burke−Ernzerhof exchange−correlation potential was used. The structures were relaxed until the forces were smaller than 0.05 eV Å−1. (b,c) Reprinted with permission from ref 29 (copyright 2012, John Wiley and Sons); (d) reprinted with permission from ref 32 (copyright 2010, Nature Publishing Group); and (e) reprinted with permission from ref 33.
or atomic shuttling around the molecular skeleton,31 remains unresolved at this stage, and future work is called for. The double-decker phthalocyanine like TbPc2 is composed of a metal cation (Tb) sandwiched between two Pc ligands (Figure 1d).32 As can be seen from the simulated STM image (upper panel of Figure 1d), the top and bottom Pc ligand deviate from each other at an angle of 45°, producing an achiral eight-lobe topography (middle in Figure 1e). The further ∼4° clockwise (counterclockwise) rotation of the top Pc ligand creates R- (L-) type distortion. The chiral feature can be easily distinguished by the symmetry reduction in STM images of Figure 1e. The chiral switching is controlled by the STM tip pulse as the combined result of inelastic electron tunneling and local current heating.33 Besides these molecular switches activatable by the voltage pulse, the charged phthalocyanine molecules subjected to Jahn−Teller (JT) distortion also show the ability to switch.34 Negatively charged copper(II) phthalocyanine (CuPc) on NaCl/Cu(100) acts as a bistable switch when its energetic order is tuned via manipulation of the adjacent Au or Ag atoms. The operation of this type of molecular switches is based on
atom, for example, hydrogen (H2) or various metal ions, has two metastable locations to stay. The switching can be accomplished either by tautomerizing the inner hydrogen atom (naphthalocyanine27) or by pulling and pushing the central atom from the original molecule plane (tin and lead phthalocyanine, SnPc and PbPc26). Besides, there is another type of Pc molecule that is inherently dipolar, such as chloroaluminum (ClAlPc)29 and vanadyl phthalocyanine (VOPc).30 The Cl or O anion protrudes out of the molecular plane carrying a permanent dipole. Such dipoles can be aligned unidirectionally when the molecules are deposited on the relatively inert graphite surface with a face-up orientation (Cl or O pointing toward vacuum). The reversible switching of ClAlPc, for instance, between the Cl-up and Cl-down (denoted as 1 and 0, respectively) configurations is realized by applying a pulse voltage to the STM tip.29 As depicted in Figure 1, the ClAlPc is switched from 1 to 0 by a +4.5 V/10 ms pulse with the tip held at +4.5 V/150 pA and back to 1 at a negative pulse (−3 V/2 ms). These manipulations can be performed at either 4 or 77 K.29 The switching mechanism, whether it is atomic tunneling, molecular flipping, 4096
dx.doi.org/10.1021/jz402080f | J. Phys. Chem. Lett. 2013, 4, 4095−4102
The Journal of Physical Chemistry Letters
Perspective
Figure 2. Reversible spin control of the central metal ion. (a) Schematic representation of the switching process of CoTPP by adsorption and desorption of NO. (Top) Switching on: The CoTPP molecule is ferromagnetically coupled to the Ni substrate, and the Co magnetic moment follows the substrate (Ni) magnetization. (Bottom) Switching off: After the addition of NO, CoTPP (S = 1/2) forms the NO−CoTPP complex (S = 0), and the spin state of NO−CoTPP remains the same. Reversibility is shown by the reaction arrows indicating the chemical reaction with NO and the dissociation of NO. (b) STM image and dI/dV spectrum of the MnPc molecule before and after CO desorption. (c) STM simulation and PDOS of the MnPc/Au(111) before and after hydrogen adsorption. Their corresponding PDOS of orbitals of Mn ion in the MnPc/Au(111) and the H-MnPc/Au(111) are depicted in the right panels, revealing changing of the spin state of the Mn ion. The DFT calculations were performed using VASP, the Perdew−Wang exchange−correlation function, and the projector-augmented wave method. The energy cutoff for the plane-wave basis set was 400 eV. A c(7 × 8) supercell containing three layers of gold atoms.37 (a) Reprinted with permission from ref 36 (copyright 2010, Nature Publishing Group); (b) reprinted with permission from ref 35 (copyright 2012, American Physical Society); (c) reprinted with permission from ref 37 (copyright 2010. Nature Publishing Group).
unpaired electron with S = 1/2.36 It can be reversibly toggled off/on by NO adsorption/desorption through the so-called trans-effect. Interfacial coordination chemistry demonstrated that the introduction of additional ligand at the trans-site might lead to the instability of the original one. For CoTPP/Ni(001), the nickel substrate establishes a certain ligand with the central Co ion in the absence of NO. When Co absorbs one NO, it is slightly drawn out of the molecular plane. Accordingly, the coupling between the Co and Ni substrate is weakened. As depicted in Figure 2a, the NO contributes one electron to the singly occupied molecular orbital (SOMO) of CoTPP (dz2); thereby, the spin state of the NO−CoTPP complex becomes 0, the off state. It can be switched on with S = 1/2 after desorption of NO. The spin states of surface-supported metal phthalocyanines can also be individually manipulated at the atomic level with the STM tip. As reported by Strózecka et al. recently,35 the spin state of manganese phthalocyanine (MnPc) on Bi(110) can be reversibly changed by decoration of a CO molecule on the central Mn2+ ion. The dangling bond of the substrate bismuth atom can be passivated by forming a local Mn−Bi bond, which brings the Mn spin from 3/2 to 1. After the dose of CO gas, the MnPc molecule exhibits a central bright protrusion of ∼0.8 Å due to the single CO bonding with Mn (Figure 2b). This variation can be checked spectroscopically by the zero-bias resonance (ZBR, the dip at zero bias), with its width being reversibly tunable by the STM tip pulse (the STS curve in Figure 2b). However, in contrast to the above-discussed trans-effect, CO is anchored to
their nonuniform charge distribution, which is sensitive to the local electric field. Specifically, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of CuPc are delocalized over the phthalocyanine lobes. Among them, the LUMOs are two-fold-degenerate eg orbitals. This degeneracy will be lifted by JT distortion once the molecule is negatively charged, resulting in a nonuniform charge distribution within CuPc and symmetry breaking from four- to two-fold symmetry. In order to change the orbital orientation, a negatively charged Au ion or neutral Ag atom is necessary to be placed nearby and manipulated so as to disturb the orbital orientation. In view of this, the JT-based molecular switch is controlled by changing the local environment through the Au or Ag atoms in its vicinity. The above-mentioned molecular switches are generally controlled by manipulating the molecule or adjacent atoms with the STM tip, in which the conformation of the molecules can be reversibly adjusted. Another type of molecular switch, on the other hand, functions as varying its spin or charge state simply by adsorption/desorption of gaseous molecules without an obvious change in molecular conformation. Spin State within a Single Molecule. It is known that the central metal ion in the metal phthalocyanine and metal porphyrin generally has unsaturated coordination, and its spin state might be altered by external chemical adsorbates such as carbon monoxide (CO),35 nitric oxide (NO)36, and a hydrogen atom (H).37 For example, the spin state of cobalt(II)tetraphenylporphyrin (CoTPP) on a Ni(001) substrate is characterized by one 4097
dx.doi.org/10.1021/jz402080f | J. Phys. Chem. Lett. 2013, 4, 4095−4102
The Journal of Physical Chemistry Letters
Perspective
Figure 3. Manipulation and control of the spin state of the Pc ligand. (a) Conversion of the center molecule from 45 to 30° by applying a current pulse. The target molecule is marked by an arrow. Changes in the contrast are schematically illustrated at the top. (b) Comparison of the Kondo peaks on the ligand before (I) and after (II) the application of the pulse. (c) STM image of single CuPc (top, I = 72 pA, V = −5 mV) and NiPc (bottom, I = 100 pA, V = −3 mV) on the Ag(100) surface. The dI/dV spectra of CuPc (top) and NiPc (bottom) around EF measured on the benzene lobe (blue dot) show the Kondo effect. (d) (Left) Top view of the differential charge density induced by CuPc bonding to the Ag(100) substrate showing the inequivalent charge distribution. The contour value is ±3.8 × 10−3 e/Å3. The excess (depletion) of charge is shown in yellow (red). (Right) Representative theoretical conductance map of the HOMO. The ab initio calculation using the VASP implementation in the projecter-augmented plane wave scheme and the local density approximation (LDA) was performed. The calculated slab included five Ag atomic layers intercalated by eight vacuum layers and a 7 × 7 lateral supercell, relaxed until forces were smaller than 0.04 eV/Å. (e) (Left) Topography images of CuPc, LiCuPc-LA (Li located at the lobe), and LiCuPc-M (Li located at the central metal) from top to bottom. (Right) dI/dV spectra taken on the benzene ring of CuPc, LiCuPc-LA, and LiCuPc-M. The set points are V = −1 V, I = 1 nA for LiCuPc-LA and LiCuPc-M and V = 2 V, I = 3 nA for CuPc. (a,b) Reprinted with permission from ref 32 (copyright 2010, Nature Publishing Group); (c) reprinted with permission from ref 40 (copyright 2011, Nature Publishing Group); (d) reprinted with permission from refs 38 and 39 (copyright 2010, American Physics Society); (e) reprinted with permission from ref 41 (copyright 2013, Nature Publishing Group).
the Mn via carbon π* backbonding. The Mn dz2 orbital overlaps with a nonbonding CO orbital and remains empty, while the dxz/dyz orbitals are fully occupied due to the hybridization with CO px and py. In this case, the spin of the CO−MnPc complex is reduced to 1/2 because of the unpaired electron in the dxy orbital. However, the surface state of the Bi substrate may get involved in the spin−orbital coupling and hence spin polarized through the Rashba effect, which would hinder the control of molecular spin. Alternatively, using the Au(111) substrate and hydrogen atom instead of CO can also gain the reversible single-spin control of MnPc. The spin state transfers between S = 3/2 and S = 1 by desorption and absorption of a H atom manipulated by the STM
tip, as shown in Figure 2c. Furthermore, this well-controlled manipulation can be conducted on the densely packed MnPc array on Au(111).37 As the phthalocyanine molecule is neutral, the number of π electrons is determined by the central metal ion. For example, TbPc2 has two Pc2− ligands (totally −4), and the central Tb ion is +3 charged; therefore, one +1 π-radical is left on the ligand.32 The molecular spin state can be switched back and forth by applying pulse voltages to rotate the upper ligand, which in turn will shift the molecular frontier orbital energy and quench the π-electron spin. The Kondo effect from upper Pc ligand can be monitored by STS featuring a Kondo peak at around 0 mV. 4098
dx.doi.org/10.1021/jz402080f | J. Phys. Chem. Lett. 2013, 4, 4095−4102
The Journal of Physical Chemistry Letters
Perspective
Figure 4. Atomic-precise control of the central metal ions of phthalocyanine. (a) (Top) Constant-current STM images (0.1 nA, 0.1 V) of H2Pc and the derived molecules. Related structures of the central cavity at different intermediate states are shown as insets. The tautomerization of H2Pc was induced by voltage pulses of 1.6 V. STM images of H-Pc obtained after removal of one pyrrolic hydrogen atom from H2Pc by a voltage pulse of 3.0 V. Hopping of the remaining pyrrolic hydrogen atom was induced by voltage pulses of 2.5 V. The Pc molecule without H atoms fabricated by pulsing the voltage to 3.5 V. After approach of the Ag tip to the center of the Pc, MPc↑ is formed. A voltage pulse of 3.0 V leads to an interconversion of MPc↑ into MPc↓. The last STM image was one AgPc↓ molecule. (Bottom) Optimized structures of (left) H2Pc and (right) AgPc in the gas phase. Calculations were performed using Gaussian03 with the ROHF/LANL2DZ basis set. (b) STM images demonstrate the topographical change before and after demetalation of the PbPc molecules marked by crosses. The Δh represents the tip retracted distance after the demetalation process. The bottom schematically shows the demetalation of a PbPc molecule. The tip moves toward the central metal ion, approaching the Pb ion, and then transfers the Pb atom to the tip apex. Scanning parameters: three top images: 4.4 nm × 3.5 nm, 0.1 V, 0.1 nA; two middle images: 12.4 nm × 5.7 nm, 0.1 V, 0.1 nA. (a) Reprinted with permission from ref 44 (Copyright 2011, Wiley-VCH, Weinheim); (b) reprinted with permission from ref 46.
The interplay between π-orbitals and upper ligand rotation can be rationalized from the relation between the TbPc2’s HOMO and SOMO. The ligand spin is quenched at a 30° upper−lower orientation, and the SOMO is doubly occupied because of charge transfer from the Au(111) substrate. The Kondo peak vanishes upon such rotation, as shown by the spectra in Figure 3b measured at the marked site in Figure 3a. As shown above, the ligand spin of metal phthalocyanine can be notably modified by charge transfer between the ligand and substrate. For instance, charges are transferred from the Ag(100) substrate to CuPc or NiPc after deposition that induces the ligand spin (DFT results in Figure 3d).39 Unpaired electrons are created in the 2eg LUMO state represented by the STS peak at zero bias (Figure 3c).40 Furthermore, this spin state can be manipulated between 0 and 1/2 by doping Li atom-by-atom.41 The Li dopants enter different ligand positions, exhibiting distinct STM topographies. As shown in Figure 3e, the Li atom can locate under the benzene ring, next to the aza-N atoms, and near the central metal atom, respectively (from top to bottom). For undoped CuPc, the Cu b1g orbital is occupied by one electron, while two electrons take the 2eg orbital in the Pc ligand; thus, the spin is SM = 1/2, SL = 1/2 (SM: metal spin; SL: ligand spin) (dI/dV spectra in Figure 3e). Doping Li transfers one electron to the 2eg orbital in LiCuPc-LA (Li at the ligand) or to the b1g orbital in LiCuPc-M, which leads to SM = 1/2, SL = 0 or SM = SL = 0 spin state, respectively (middle and bottom panels in Figure 3e). The Ag(100) substrate serves as both the electron donor and dopant acceptor; it acts like a charge sink to limit the charge uptake of MPc to Q = 2.
The molecular switch is the basic unit in molecular electronics. Many approaches can be applied to realize the on/off state by manipulating the central group and/or the peripheral ligand of the phthalocyanine by STM. Atomically Precise Surface Reaction. STM has been applied to the surface reaction at the single-atom and single-molecule levels.42 The metalation and demetalation of the phthalocyanine molecule are of particular interest in terms of modifying the central metal atom to trigger the molecular switch. Metalation of H2Pc has been achieved on the Pb(111) thin film by depositing iron atoms.43 However, it can barely provide single-molecule accuracy but a statistical distribution of the metalated phthalocyanine (FePc). As an alternative, precise metalation of single H2Pc to AgPc can be attained by STM manipulation.44 Starting from H2Pc, the hydrogen tautomerization is first triggered by a moderate tip pulse then deprotonation at elevated voltages (H-Pc). The hopping of a single hydrogen atom between four adjacent sites within the central cavity serves as a four-level conductance switch.45 Increasing the tip pulse higher to completely remove the hydrogen gives rise to an empty cavity that is reactive with the metal atom from the STM tip when the tip is at close proximity. Figure 4a shows the deprotonation process of H2Pc and the formation of metalPc by injecting one Ag atom into the central cavity. A variety of 4099
dx.doi.org/10.1021/jz402080f | J. Phys. Chem. Lett. 2013, 4, 4095−4102
The Journal of Physical Chemistry Letters
Perspective
Figure 5. Imaging the charge distribution within a single molecule. (a) STM image (I = 2 pA, V = 0.25 V) of naphthalocyanine on NaCl(2 ML)/ Cu(111). The cross represents a single naphthalocyanine molecule, and the protrusion is identified as a single CO molecule. (b) High-resolution STM image of a naphthalocyanine molecule (I = 2 pA, V = 0.6 V) obtained by a CO-terminated tip. The positions of the central hydrogens and the tautomerization path are highlighted in red, and the definition of the H and N lobes is illustrated. (c,d) Constant-height AFM frequency shift images of the same molecule as that in (b), measured with a CO-terminated tip. The images were recorded at distances of z = 0.145 (c) and 0.175 nm (d) above the height determined by the STM set point (I = 2 pA, V = 0.2 V) over the substrate. (e,f) Cuts through the DFT-calculated electron density of a naphthalocyanine molecule at distances of d = 0.2 (e) and 0.3 nm (f) from the molecular plane. (g,h) Asymmetry of the calculated electron density at d = 0.1 (g) and 0.4 nm (h) from the molecular plane. (i,j) LCPD images of naphthalocyanine on NaCl(2 ML)/Cu(111) before (i) and after (j) switching the tautomerization state of the molecule. The images were recorded with a copper-terminated tip on a 64 × 64 lateral grid at constant height (z = 0.1 nm above the height determined by the STM set point (I = 3 pA, V = 0.2 V) over the substrate). (k) Difference image obtained by subtracting (j) from (i). (l) DFT-calculated asymmetry of the z-component of the electric field above a free naphthalocyanine molecule at a distance d = 0.5 nm from the molecular plane. Scale bars: 2 nm in (a) and 0.5 nm elsewhere. DFT calculations were performed for the free naphthalocyanine molecule using the highly optimized plane wave code CPMD. The Perdew−Burke−Ernzerhof exchange−correlation functional and ab initio norm-conserving pseudopotentials were used. The size of the unit cell was 3.2 × 3.2 × 1.6 nm3. Structural optimization was performed until the forces on all atoms were below 1 × 10−3 eV nm−1, and a cutoff energy of 2 keV was used for a single k point. Reprinted with permission from ref 6 (Copyright 2012, Nature Publishing Group).
molecules.47 KPFM measures the local contact potential difference (LCPD) by applying a voltage across the sample and an AFM tip with submolecular resolution.6 To demonstrate such capabilities, a single naphthalocyanine molecule on NaCl(2 ML)/Cu(111) is an ideal model benefiting from the tautomerization of two inner hydrogen atoms. The N and H lobes of the naphthalocyanine are distinctly identified by STM to show a two-fold symmetry with a nodal plane along the N lobe (Figure 5a and b). Then, a CO-terminated AFM tip is used to acquire atomic resolution of C6 rings within each lobe (Figure 5c and d). The topographic contrast is associated with the electron density as a function of the tip−molecule distance. As shown in Figure 5i and j, the LCPD images recorded before and after switching the tautomerization exhibit a distinct asymmetric charge distribution between the H and N lobes. The measured KPFM signal appears to be completely independent of the tip current and frequency shift; hence, it can be concluded that the charge distribution within the naphthalocyanine is the primary cause of the KPFM contrast. The total electron density or the electric field profile could be further derived by combining KPFM with AFM.
intermediates are clearly visible in the sequential STM topographies. The voltage thresholds for complete dehydrogenation and interconversion of the central metal atom are related to the filling of electrons in the unoccupied orbital coupled with the molecular vibration. Reversely, the demetalation of single PbPc on the ultrathin Pb film on Ag(111) has also been successfully achieved by transferring the central Pb atom to the apex of the STM tip.46 It requires the injection of electrons into the unoccupied molecular state with a tip bias of ∼2.2 V along with the vibrational excitation. The demetalation process is demonstrated in Figure 4b. The molecules with bright protrusion are PbPc, while those with dark center are the demetalated molecules. The handy metalation and demetalation provide a feasible approach to precise surface reaction and tailoring of the properties of the molecular arrays to probe their mutual interactions. In addition, the metalated and demetalated molecule can be treated as two states of the molecular switch. The switching rate depends strongly on both the pulse voltage and the tip position.12 In order to uncover the mechanism, it is necessary to map out the charge distribution within each molecule that is closely related to the bonding and hence switching.29 Imaging the Charge Distribution of Individual Molecules. SPM is widely used to image the spatial distribution of topographic and electronic properties of single molecules at the atomic level. Direct imaging of the charge density is of great interest in the fundamental insights into the single-molecule switching and the bond formation between atoms and molecules.7 Particularly, it helps to understand the interactive role of a specific working environment such as the substrate underneath of the monolayer
Atomic and bond precision revealing the charge distribution within single molecules can provide fundamental insight into the molecular switches and on-site surface reactions. 4100
dx.doi.org/10.1021/jz402080f | J. Phys. Chem. Lett. 2013, 4, 4095−4102
The Journal of Physical Chemistry Letters
Perspective
Characterization of the interfacial properties of single molecules on a supporting substrate by means of SPM provides remarkably rich information at the atomic level in the field of molecular electronics. The advancement in SPM technology offers a versatile tool to visualize the atomic structure, charge distribution, and bond formation within a single molecule. It allows the capture of the conformational changes and the surface reaction process of molecules on the electrode, which are essential for designing and fabricating molecular electronic devices. There are, however, grand challenges in technical aspects to bring the molecular electronic devices into practice. Wiring single molecules into circuits and stabilizing the functional units under ambient conditions, for example, are still crucial issues. The SPM methods reviewed in this Perspective are mostly carried out at low temperatures to ensure the molecular stability. One probable solution to stabilize the molecules on electrodes would be covalently linking them together without noticeably disturbing the intrinsic properties of molecules. In situ STM investigation of the on-surface polymerization of functional molecules on arbitrary substrates has demonstrated such a promising route toward the enhancement of the efficient coupling between molecules and substrates. Besides the fundamental physics of the single molecules and the hybrid organic−inorganic heterostructures, one major topic of SPM study on single organic molecules is the in situ on-surface reaction under control. Fabricating molecular electronics strongly relies on the precise control of the physical and chemical properties of the molecular junctions and implementing these functional units into the circuits accurately and reproducibly. The development of technologies that aim to scale down the electronic devices made it possible to gain atomic-level control of the interfaces between basic components and the electrodes. However, the commercialization of molecular electronics is still a long way away. Furthermore, using atoms instead of organic molecules could raise the conductance and shrink the unit size, making the implementation of the molecular switches easier and more repeatable. As we have demonstrated, the fundamental insight into the properties of molecules or atoms by means of SPM is so fruitful to pave the way to practical molecular devices. It calls for innovative hybrid structures, new electrode materials, and proper tooling for characterization/manipulation of quantum charge and spin properties.
■
he joined the Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences as a full professor. His work is focused on a developing low-temperature, ultrahigh vacuum scanning tunneling microscope and conducting fundamental research with that tool on strongly correlated electron systems such as hightemperature superconductors.
■
ACKNOWLEDGMENTS The research described herein was supported by the State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences and the Surface Science Lab, Department of Physics, National University of Singapore. T.C.N. would like to acknowledge his Ph.D. thesis advisor A/P Wei Chen and all group members in NUS for their contributions to the STM study on phthalocyanine molecules.
■
REFERENCES
(1) Aradhya, S. V.; Venkataraman, L. Single-Molecule Junctions beyond Electronic Transport. Nat. Nanotechnol. 2013, 8, 399−410. (2) Barth, J. V.; Costantini, G.; Kern, K. Engineering Atomic and Molecular Nanostructures at Surfaces. Nature 2005, 437, 671−679. (3) Weigelt, S.; Busse, C.; Petersen, L.; Rauls, E.; Hammer, B.; Gothelf, K. V.; Besenbacher, F.; Linderoth, T. R. Chiral Switching by Spontaneous Conformational Change in Adsorbed Organic Molecules. Nat. Mater. 2006, 5, 112−117. (4) de Oteyza, D. G.; Gorman, P.; Chen, Y.-C.; Wickenburg, S.; Riss, A.; Mowbray, D. J.; Etkin, G.; Pedramrazi, Z.; Tsai, H.-Z.; Rubio, A.; Crommie, M. F.; Fischer, F. R. Direct Imaging of Covalent Bond Structure in Single-Molecule Chemical Reactions. Science 2013, 340, 1434−1437. (5) Chen, W.; Li, H.; Huang, H.; Chen, S.; Huang, Y. L.; Gao, X. Y.; Wee, A. T. S. Two-Dimensional Pentacene:3,4,9,10-Perylenetetracarboxylic Dianhydride Supramolecular Chiral Networks on Ag(111). J. Am. Chem. Soc. 2008, 130, 12285−12289. (6) Mohn, F.; Gross, L.; Moll, N.; Meyer, G. Imaging the Charge Distribution within a Single Molecule. Nat. Nanotechnol. 2012, 7, 227−231. (7) Friesen, B. A; Bhattarai, A.; Mazur, U.; Hipps, K. W. Single Molecule Imaging of Oxygenation of Cobalt Octaethylporphyrin at the Solution/Solid Interface: Thermodynamics from Microscopy. J. Am. Chem. Soc. 2012, 134, 14897−14904. (8) Zhao, A.; Tan, S.; Li, B.; Wang, B.; Yang, J. L.; Hou, J. G. STM Tip-Assisted Single Molecule Chemistry. Phys. Chem. Chem. Phys. 2013, 15, 12428−12441. (9) Boer, D.; Li, M.; Habets, T.; Iavicoli, P.; Rowan, A. E.; Nolte, R. J. M.; Speller, S.; Amabilino, D. B.; Feyter, S.; Elemans, J. A. A. W. Detection of Different Oxidation States of Individual Manganese Porphyrins during Their Reaction with Oxygen at a Solid/Liquid Interface. Nat. Chem. 2013, 5, 621−627. (10) Huang, Y. L.; Chen, W.; Wee, A. T. S. Molecular Trapping on Two-Dimensional Binary Supramolecular Networks. J. Am. Chem. Soc. 2011, 133, 820−825. (11) Bao, B. Q.; Tao, N. J. Measurement of Single-Molecule Resistance by Repeated Formation of Molecular Junctions. Science 2003, 301, 1221−1223. (12) Vincent, R.; Klyatskaya, S.; Ruben, M.; Wernsdorfer, W.; Balestro, F. Electronic Read-out of a Single Nuclear Spin Using a Molecular Spin Transistor. Nature 2012, 488, 357−360. (13) Focus Issue. Nat. Nanotechnol. 2013, 8, 377. (14) Aradhya, S. V.; Venkataraman, L. Single-Molecule Junctions beyond Electronic Transport. Nat. Nanotechnol. 2013, 8, 399−410. (15) Barth, J. V.; Costantini, G.; Kern, K. Engineering Atomic and Molecular Nanostructures at Surfaces. Nature 2005, 437, 671−679. (16) Gopakumar, T. G.; Tang, H.; Morillo, J.; Berndt, R. Transfer of Cl Ligands between Adsorbed Fe-Tetraphenylporphyrin Molecules. J. Am. Chem. Soc. 2012, 134, 1844−11847.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. Biographies Tianchao Niu received his M.S. from Soochow University in 2006, and he finalized his Ph.D. at the National University of Singapore in July 2013 under the supervision of Associate Professor Wei Chen. He is currently an Assistant Professor in the Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences. His research focuses on the construction of 2D molecular dipole arrays, exploring and manipulating single molecules by using STM, and also the study of the graphene growth mechanism based on the in situ CVD method combined with LT-STM. Ang Li received his Ph.D. in Condensed Matter Physics from Nanjing University in 2002. He continued as a postdoctoral fellow at the Texas Center for Superconductivity at the University of Houston. In 2011, 4101
dx.doi.org/10.1021/jz402080f | J. Phys. Chem. Lett. 2013, 4, 4095−4102
The Journal of Physical Chemistry Letters
Perspective
(17) Pavliček, N.; Fleury, B.; Neu, M.; Niedenführ, J.; HerranzLancho, C.; Ruben, M.; Repp, J. Atomic Force Microscopy Reveals Bistable Configurations of Dibenzo[a,h]thianthrene and Their Interconversion Pathway. Phys. Rev. Lett. 2012, 108, 086101. (18) Melitz, W.; Shen, J.; Kummel, A. C.; Lee, S. Kelvin Probe Force Microscopy and Its Application. Surf. Sci. Rep. 2011, 66, 1−27. (19) Gross, L. Recent Advances in Submolecular Resolution with Scanning Probe Microscopy. Nat. Chem. 2011, 3, 273−278. (20) Walter, M. G.; Rudine, A. B.; Wamser, C. C. Porphyrins and Phthalocyanines in Solar Photovoltaic Cells. J. Porphyrins Phthalocyanines 2010, 14, 759. (21) Li, Z.; Li, B.; Yang, J.; Hou, J. G. Single-Molecule Chemistry of Metal Phthalocyanine on Noble Metal Surfaces. Acc. Chem. Res. 2010, 43, 954−962. (22) Schmaus, S.; Bagrets, A.; Nahas, Y.; Yamada, T. K.; Bork, A.; Bowen, M.; Beaurepaire, E.; Evers, F.; Wulfhekel, W. Giant Magnetoresistance through a Single Molecule. Nat. Nanotechnol. 2011, 6, 185−189. (23) Zhang, Y. Y.; Du, S. X.; Gao, H.-J. Binding Configuration, Electronic Structure, and Magnetic Properties of Metal Phthalocyanines on a Au(111) Surface Studied with Ab Initio Calculations. Phys. Rev. B 2011, 84, 125446. (24) Minamitani, E.; Tsukahara, N.; Matsunaka, D.; Kim, Y.; Takagi, N.; Kawai, M. Symmetry-Driven Novel Kondo Effect in a Molecule. Phys. Rev. Lett. 2012, 109, 086602. (25) Gao, L.; Liu, Q.; Zhang, Y. Y.; Jiang, N. H.; Zhang, G.; Cheng, Z. H.; Qiu, W. F.; Du, S. X.; Liu, Y. Q.; Hofer, W. A.; Gao, H.-J. Constructing an Array of Anchored Single-Molecule Rotors on Gold Surfaces. Phys. Rev. Lett. 2008, 101, 197209. (26) Wang, Y. F.; Kröger, J.; Berndt, R.; Hofer, W. A. Pushing and Pulling a Sn Ion through an Adsorbed Phthalocyanine Molecule. J. Am. Chem. Soc. 2009, 131, 3639−3643. (27) Liljeroth, P.; Repp, J.; Meyer, G. Current-Induced Hydrogen Tautomerization and Conductance Switching of Naphthalocyanine Molecules. Science 2007, 317, 1203−1206. (28) Weinelt, M.; Oppen, F. Molecular Switches at Surfaces. J. Phys.: Condens. Matter 2012, 24, 390201. (29) Huang, Y. L.; Lu, Y. H.; Niu, T. C.; Huang, H.; Kera, S.; Ueno, N.; Wee, A. T. S.; Chen, W. Reversible Single-Molecule Switching in an Ordered Monolayer Molecular Dipole Array. Small 2012, 8, 1423− 1428. (30) Niu, T. C.; Zhou, C. G.; Zhang, J. L.; Zhong, S.; Cheng, H. S.; Chen, W. Substrate Reconstruction Mediated Unidirectionally Aligned Molecular Dipole Dot Arrays. J. Phys. Chem. C 2012, 116, 11565− 11569. (31) Zhang, J. L.; Xu, L. J.; Niu, T. C.; Lu, Y. H.; Liu, L.; Chen, W. Reversible Switching of Single Dipole Molecule Imbedded in Two Dimensional Hydrogen-Bonded Binary Molecular Networks. J. Phys. Chem. C, submitted. (32) Komeda, T.; Isshiki, H.; Liu, J.; Zhang, Y. F.; Lorente, N.; Katoh, K.; Breedlove, B. K.; Yamashita, M. Observation and Electric Current Control of a Local Spin in a Single-Molecule Magnet. Nat. Commun. 2011, 2, 217. (33) Fu, Y. S.; Schwöbel, J.; Hla, S.-W.; Dilullo, A.; Hoffmann, G.; Klyatskaya, S.; Ruben, M.; Wiesendanger, R. Reversible Chiral Switching of Bis(phthalocyaninato) Terbium(III) on a Metal Surface. Nano Lett. 2012, 12, 3931−3935. (34) Uhlmann, C.; Swart, I.; Repp, J. Controlling the Orbital Sequence in Individual Cu-Phthalocyanine Molecules. Nano Lett. 2013, 13, 777−780. (35) Strózė cka, A.; Soriano, M.; Pascual, J. I.; Palacios, J. J. Reversible Change of the Spin State in a Manganese Phthalocyanine by Coordination of CO Molecule. Phys. Rev. Lett. 2012, 109, 147202. (36) Wäckerlin, C.; Chylarecka, D.; Kleibert, A.; Müller, K.; Iacovita, C.; Nolting, F.; Jung, T. A.; Ballav, N. Controlling Spins in Adsorbed Molecules by a Chemical Switch. Nat. Commun. 2010, 1, 61. (37) Liu, L.; Yang, K.; Jiang, Y.; Song, B.; Xiao, W.; Li, L.; Zhou, H.; Wang, Y.; Du, S.; Ou, M.; Hofer, W. A.; Neto, A. H. C.; Gao, H.-J.
Reversible Single Spin Control of Individual Magnetic Molecule by Hydrogen Atom Adsorption. Sci. Rep. 2010, 3, 1210. (38) Mugarza, A.; Lorente, N.; Ordejón, P.; Krull, P. C.; Stepanow, S.; Bocquet, M.-L.; Fraxedas, J.; Ceballos, G.; Gambardella, P. Orbital Specific Chirality and Homochiral Self-Assembly of Achiral Molecules Induced by Charge Transfer and Spontaneous Symmetry Breaking. Phys. Rev. Lett. 2010, 105, 115702. (39) Mugarza, A.; Robles, R.; Krull, C.; Korytár, R.; Lorente, N.; Gambardella, P. Electronic and Magnetic Properties of Molecule− Metal Interfaces: Transition-Metal Phthalocyanines Adsorbed on Ag(100). Phys. Rev. B 2012, 85, 155437. (40) Mugarza, A.; Krull, C.; Robles, R.; Stepanow, S.; Ceballos, G.; Gambardella, P. Spin Coupling and Relaxation Inside Molecule−Metal Contacts. Nat. Commun. 2011, 2, 490. (41) Krull, C.; Robles, R.; Mugarza, A.; Gambardella, P. Site- and Orbital-Dependent Charge Donation and Spin Manipulation in Electron-Doped Metal Phthalocyanines. Nat. Mater. 2013, 12, 337− 343. (42) Hla, S.-W.; Rieder, K.-H. STM Control of Chemical Reactions: Single-Molecule Synthesis. Annu. Rev. Phys. Chem. 2013, 54, 307−330. (43) Song, C. L.; Wang, Y. L.; Ning, Y. X.; Jia, J. F.; Chen, X.; Sun, B.; Zhang, P.; Xue, Q. K.; Ma, X. Tailoring Phthalocyanine Metalation Reaction by Quantum Size Effect. J. Am. Chem. Soc. 2010, 132, 1456− 1457. (44) Sperl, A.; Kröger, J.; Berndt, R. Controlled Metalation of a Single Adsorbed Phthalocyanine. Angew. Chem., Int. Ed. 2011, 50, 5294−5297. (45) Auwärter, W.; Seufert, K.; Bischoff, F.; Ecija, D.; Vijayaraghavan, S.; Joshi, S.; Klappenberger, F.; Samudrala, N.; Barth, J. V. A SurfaceAnchored Molecular Four-Level Conductance Switch Based on Single Proton Transfer. Nat. Nanotechnol. 2011, 11, 41−46. (46) Sperl, A.; Kröger, J.; Berndt, R. Demetalation of a Single Organometallic Complex. J. Am. Chem. Soc. 2011, 133, 11007−11009. (47) Elemans, J. A. A. W.; Hameren, R.; Nolte, R. J. M.; Rowan, A. E. Molecular Materials by Self-Assembly of Porphyrins, Phthalocyanines, and Perylenes. Adv. Mater. 2006, 18, 1251−1266.
4102
dx.doi.org/10.1021/jz402080f | J. Phys. Chem. Lett. 2013, 4, 4095−4102
