The discovery of the photoelectric effect by Heinrich Hertz in 1887 set the foundation for over 125 years of hot carrier science and technology. In the early 1900s it played a critical role in the development of quantum mechanics, but even today the unique properties of these energetic, hot carriers offer new and exciting opportunities for fundamental research and applications. Measurement of the kinetic energy and momentum of photoejected hot electrons can provide valuable information on the electronic structure of materials. The heat generated by hot carriers can be harvested to drive a wide range of physical and chemical processes. Their kinetic energy can be used to harvest solar energy or create sensitive photodetectors and spectrometers. Photoejected charges can also be used to electrically dope two-dimensional materials. Plasmon excitations in metallic nanostructures can be engineered to enhance and provide valuable control over the emission of hot carriers. This Review discusses recent advances in the understanding and application of plasmon-induced hot carrier generation and highlights some of the exciting new directions for the field.

Plasmon-induced hot carrier science and technology

Optical generation of hot electrons in metallic structures and its potential as an alternative to conventional electron–hole separation in semiconductor devices are reviewed. The possibilities for realizing high conversion efficiencies with low fabrication costs are discussed along with challenges in terms of the materials, architectures and fabrication methods

Plasmon-induced hot-electron generation at nanoparticle/metal-oxide interfaces for photovoltaic and photocatalytic devices

Nanoantennas are key optical components for light harvesting; photodiodes convert light into a current of electrons for photodetection. We show that these two distinct, independent functions can be combined into the same structure. Photons coupled into a metallic nanoantenna excite resonant plasmons, which decay into energetic, "hot" electrons injected over a potential barrier at the nanoantenna-semiconductor interface, resulting in a photocurrent. This dual-function structure is a highly compact, wavelength-resonant, and polarization-specific light detector, with a spectral response extending to energies well below the semiconductor band edge.

http://science.sciencemag.org/content/sci/332/6030/702.full.pdf

Photodetection with Active Optical Antennas

Noble metal particles have long fascinated scientists because of their intense color, which led to their application in stained glass windows as early as the Middle Ages. The recent resurrection of colloidal and cluster chemistry has brought about the strive for new materials that allow a bottoms-up approach of building improved and new devices with nanoparticles or artificial atoms. In this review, we discuss some of the properties of individual and some assembled metallic nanoparticles with a focus on their interaction with cw and pulsed laser light of different energies. The potential application of the plasmon resonance as sensors is discussed.

Optical properties and ultrafast dynamics of metallic nanocrystals.

Heterogeneous catalysis is of paramount importance in chemistry and energy applications. Catalysts that couple light energy into chemical reactions in a directed, orbital-specific manner would greatly reduce the energy input requirements of chemical transformations, revolutionizing catalysis-driven chemistry. Here we report the room temperature dissociation of H2 on gold nanoparticles using visible light. Surface plasmons excited in the Au nanoparticle decay into hot electrons with energies between the vacuum level and the work function of the metal. In this transient state, hot electrons can transfer into a Feshbach resonance of an H2 molecule adsorbed on the Au nanoparticle surface, triggering dissociation. We probe this process by detecting the formation of HD molecules from the dissociations of H2 and D2 and investigate the effect of Au nanoparticle size and wavelength of incident light on the rate of HD formation. This work opens a new pathway for controlling chemical reactions on metallic catalysts.

Hot Electrons Do the Impossible: Plasmon-Induced Dissociation of H2 on Au

Multiphoton photoelectron spectroscopy reveals the multiple excitation of the surface plasmon in silver nanoparticles on graphite. Resonant excitation of the surface plasmon with 400 nm femtosecond radiation allows one to distinguish between photoemission from the nanoparticles and the substrate. Two different previously unobserved decay channels of the collective excitation have been identified, namely, decay into one or several single-particle excitations.

Surface plasmon dynamics in silver nanoparticles studied by femtosecond time-resolved photoemission.

Multiphoton photoemission spectroscopy of graphite using 267 nm (4.65 eV) and 400 nm (3.1 eV) excitation wavelength reveals spectroscopic features that allow the identification of the multiphoton excitation process and that correspond to the known bulk band structure. In addition, the $n=1$ and $n=2$ image potential states on graphite are identified, with binding energies of 0.85 and 0.15 eV, respectively. They are characterized by a vanishing quantum defect and are located close to the top of the band gap in the projected bulk band structure. Accordingly, the $n=1$ image potential state and the minimum of the interlayer band are both located about 4 eV above the Fermi level. This settles the ambiguities in the interpretation of the unoccupied band structure of graphite with respect to the energetic location of the interlayer band. Time-resolved two-photon photoemission spectroscopy yields a lifetime of $40\ifmmode\pm\else\textpm\fi{}6\mathrm{fs}$ for the $n=1$ image potential state. This rather long lifetime of an image potential state at the top of the band gap and the vanishing quantum defect are attributed to the two-dimensional structure of graphite.

Properties and dynamics of the image potential states on graphite investigated by multiphoton photoemission spectroscopy

Time-resolved multiphoton photoelectron spectroscopy is employed to study collective excitations and their decay dynamics in silver nanoparticles on highly oriented pyrolytic graphite. Resonant excitation of the surface plasmon in the silver nanoparticles with 400 nm femtosecond radiation allows to distinguish between photoemission from the nanoparticles and the substrate. This extends the method of time-resolved two-photon photoemission spectroscopy to inhomogeneous surfaces and permits to probe the dynamics of a confined electron gas. The multiphoton photoelectron spectra, the polarization dependence of the photoelectron yield and the time-resolved measurements reveal the double excitation of the surface plasmon and allow the identification of two different decay channels of the collective excitation. The multiply excited plasmon transfers its total excitation energy to a single photoelectron or decays into at least two single-particle excitations which share the total energy.

Silver nanoparticles on graphite studied by femtosecond time-resolved multiphoton photoemission

. Time-resolved two-color two-photon photoemis-sion experiments are used to investigate the electronic dy-namics in highly oriented pyrolytic graphite (HOPG)andinAg nanoparticles grown on HOPG. The multiphoton photo-emission using 267nm and 400nm excitation is presentedand discussed. For the ﬁrst time the known unoccupied stateon graphite 3 : 6eVabove the Fermi level can be identiﬁed asan image potential state. A lower limit of 60fs is given forthe lifetime of this state. The two-color experiments revealthat the carrier relaxation deduced from time-resolved experi-mentsis inﬂuencedby feedingof the intermediatestates. Firstresults of the strongly enhanced multiphoton photoemissionfrom Ag nanoparticles on HOPG at 400nm wavelength arepresented. This observation is explained by the excitation ofthe surface plasmon and its subsequent decay.PACS: 78.47.+p; 79.60.Jv; 79.60.-i; 82.65.JvGraphite and metallic nanoparticles grown on graphite arevery interesting systems for nonlinear optical studies usingtwo-photon photoemission spectroscopy (2PPES) and itstime-resolved counterpart. The electron dynamics of the puregraphite surface show distinct differences to that of a three-dimensional electron gas system like bulk metals, whichhave already been thoroughly investigated by time-resolved2PPES [1–4]. The interpretation of time-resolved 2PPES re-sults by Xu et al. [5] has recently initiated a controversyconcerning the hot carrier relaxation mechanisms in graph-ite. The authors explain the observed deviation from standardFermi liquid theory [6] by an additional plasmon mediatedrelaxation mechanism. However, this interpretation has beenquestioned [7] and the debate is still going on. Experimentalevidencelike the two-colortime-resolved2PPES experimentspresented here provides further substantial information.Another important issue is related to the study of theunoccupied states of graphite. With the development of in-verse photoemission spectroscopy (IPES) and target currentspectroscopy (TCS) the unoccupied states of graphite wereinvestigated [8–10]. Good agreement with theory was foundfor most of the observed states [11]. However, the assign-ment ofone state about3

Time-resolved two-photon photoemission spectroscopy of HOPG and Ag nanoparticles on HOPG

Time-resolved multiphoton photoelectron spectroscopy using resonant excitation of the surface plasmon in nanoparticles permits the investigation of the electron dynamics in silver nanoparticles grown on graphite. The multiphoton photoelectron spectra, the polarization dependence of the photoelectron yield and the time-resolved measurements reveal the double excitation of the surface plasmon. Time-resolved spectroscopy gives information on the electronic temperature and the electron-phonon coupling in the nanoparticles.

J. Lehmann

Papers

Surface plasmon dynamics in silver nanoparticles studied by femtosecond time-resolved photoemission.

Properties and dynamics of the image potential states on graphite investigated by multiphoton photoemission spectroscopy

Silver nanoparticles on graphite studied by femtosecond time-resolved multiphoton photoemission

Time-resolved two-photon photoemission spectroscopy of HOPG and Ag nanoparticles on HOPG

Femtosecond multiphoton photoemission of silver nanoparticles on graphite