Photosynthesis is used by plants, algae and bacteria to convert solar energy into stable chemical energy. The initial stages of this process--where light is absorbed and energy and electrons are transferred--are mediated by reaction centres composed of chlorophyll and carotenoid complexes. It has been previously shown that single small molecules can be used as functional components in electric and optoelectronic circuits, but it has proved difficult to control and probe individual molecules for photovoltaic and photoelectrochemical applications. Here, we show that the photocurrent generated by a single photosynthetic protein-photosystem I-can be measured using a scanning near-field optical microscope set-up. One side of the protein is anchored to a gold surface that acts as an electrode, and the other is contacted by a gold-covered glass tip. The tip functions as both counter electrode and light source. A photocurrent of ∼10 pA is recorded from the covalently bound single-protein junctions, which is in agreement with the internal electron transfer times of photosystem I.

Abstract. The photoconductance properties of photosystem I (PSI) covalently bound to carbon nanotubes (CNTs) are measured. We demonstrate that the PSI forms active electronic junctions with the CNTs enabling control of the CNTs photoconductance by the PSI. In order to electrically contact the photoactive proteins, a cysteine mutant is generated at one end of the PSI by genetic engineering. The CNTs are covalently bound to this reactive group using carbodiimide chemistry. We detect an enhanced photoconductance signal of the hybrid material at photon wavelengths resonant to the absorption maxima of the PSI compared to nonresonant wavelengths. The measurements prove that it is feasible to integrate photosynthetic proteins into optoelectronic circuits at the nanoscale.

We optoelectronically functionalize carbon nanotubes (CNTs) with the photosynthetic reaction center photosystem I (PSI) according to three different on-chip chemical routes. The PSI is bound to the CNTs via covalent, hydrogen, or electrostatic bonds. Our approach allows the electrical contact of single PSI-CNT hybrid systems where the orientation of the PSI with respect to the CNTs depends on the binding mechanism. Our data are consistent with the interpretation that if the PSI is anchored with its internal electron transport path perpendicular to CNTs, the optical excitation of the PSI leads to an enhanced photoconductance of the hybrid system.

The photocurrent properties of freely suspended single-walled carbon nanotubes (CNTs) are investigated as a function of uniaxial strain. We observe that at low strain, the photocurrent signal of the CNTs increases for increasing strain, while for large strain, the signal decreases, respectively. We interpret the non-monotonous behavior by a superposition of the influence of the uniaxial strain on the resistivity of the CNTs and the effects caused by Schottky contacts between the CNTs and the metal contacts. Corresponding author: holleitner@wsi.tum.de PACS 73.22.-f, 78.67.Ch, 85.60.-q 2 Carbon nanotubes (CNTs) have attracted considerable attention because of their compelling optoelectronic [1]-[8] and electro-mechanical properties. [9]-[29] For instance, laser-induced excitonic transitions can give rise to a photoconductance of CNTs. [3],[4] A photoconductance can also be bolometrically induced in CNTs, [6],[8] and surface states due to adsorbates can alter the photoconductance of CNTs by laser-excited photodesorption of the molecules. [2] Furthermore, electric fields at the Schottky contacts between CNTs and metal contacts can separate optically excited electron-hole pairs, causing a photocurrent across electrically contacted CNTs. [5]-[7]At the same time, the electro-mechanical properties of CNTs have been studied both by locally manipulating CNTs with the tip of an atomic force microscope (AFM), [12]-[14] and by applying uniaxial [15]-[18] and torsional [19], [20] strain to the CNTs. Theorists have modeled the electronic behavior of the mechanically deformed CNTs by an enhanced electronic scattering at defects, [13],[21],[22] a structural induced alteration of the CNTs' band gap, [15]-[19],[21],[23]-[25], [29] and by a mechanical induced transition from sp 2 to sp 3 hybridization of the carbon bonds. [11],[28] Here, we report on the photocurrent properties of freely suspended CNTs as a function of statically applied uniaxial strain. To this end, we experimentally verify that the photocurrent is generated at Schottky contacts between the freely suspended CNTs and their bracing source and drain electrodes. Then, the strain is induced by applying a voltage to a piezoelectric stack, such that the distance of the source and drain electrodes is increased. We observe a rise of the photocurrent signal of up to ~150 % for uniaxial strain values in the range of 0.3 to 1.2 % and a decrease of the photocurrent signal for

We present a chemical route to covalently couple the photosystem I (PS I) to carbon nanotubes (CNTs). Small linker molecules are used to connect the PS I to the CNTs. Hybrid systems, consisting of CNTs and the PS I, promise new photo-induced transport phenomena due to the outstanding optoelectronic properties of the robust cyanobacteria membrane protein PS I