This journal is © The Royal Society of Chemistry 2016 J. Mater. Chem. C Cite this: DOI: 10.1039/c6tc04243k Plasmon induced ultrafast injection of hot electrons in Au nanoislands grown on a CdS film† Alka Sharma, ab Chhavi Sharma, ab Biplab Bhattacharyya, ab Kaweri Gambhir, ab Mahesh Kumar, ab Suresh Chand, ab Ranjana Mehrotra ab and Sudhir Husale* ab Metal nanoparticle semiconductor heterostructures exhibit unique optoelectronic properties and have potential applications in energy harvesting, photodetectors, photocatalysts, and optoelectronic devices. The hot carriers are formed at the metal nanostructure semiconductor interface, and their efficient injection into the surrounding media (semiconductor) is a great challenge and understanding the physics behind the charge transfer is currently a topic of global research. Herein, we report the investigation on the hot electron injection in the Au nanoislands formed on a CdS film using ultrafast femtosecond spectroscopy. When the Au–CdS film is excited with visible light (above the band gap), a complete bleaching effect is observed. However, when it is excited with the photon energies below the band gap of the CdS film, an absorption signal is observed over a broad spectral range. The ultrafast charge transfer dynamics studied herein indicate the possibility of the plasmons formed in the Au nanoislands, which directly decay nonradiatively by injecting electrons in the conduction band of the CdS film and charge recombination with the Au nanoislands. Finally, we demonstrate the charge transfer in the metal semiconductor hybrid, which exhibits a significant alteration in the ultrafast optical properties compared to its individual components and is dependent on the excitation energy and can thus be exploited in the light harvesting devices. Introduction The creation, detection, and efficient injection of hot electrons from metal nanostructures into a coupled media (semiconductors) have potential applications in solar energy harvesting, 1 photodetectors, 2 photocatalysts, 3 solid state lasers, 4 Schottky barriers, 5 medicine, 6 waveguide plasmon polaritons, 7 LEDs, 8 photodiodes, and electronic devices 9 due to the extraordinary properties of the metal nano- structures, which include strong scattering cross-sections, photo- thermal effect, exciton induced transparency, 10 Fano effect, 11 long range absorption that enables tuning of the band gap in the UV-vis-NIR region, 12 enhanced local electromagnetic field, and localized surface plasmon resonance. 13 The modulation of light at the nanoscale level has been presented in various studies in the past few years. It covers a wide range of heterostructures based on different materials such as metal–semiconductors, metal-oxide–semiconductors, 14 metal spacer/semiconductors, 3 which include all the epitaxial and non-epitaxial grown (Au) metal nanoparticles on the semiconductor surface, such as metal core–semiconductor shells, 15 and metal tips on the semi- conductor surface, 16 metal cages grown on the semiconductor quantum dots, 17 metal–semiconductor dumbbells, 18 and rods, 19 depending on the growth technique. The plasmonic properties of the metal nanostructures mainly depend on the size, shape, and material used along with the strong interaction between the plasmon and excitons, 12 which facilitate the metal–semiconductor based structures as promising candidates for technology oriented applications and thus it opens new opportunities for the design of hybrid devices with an enhanced efficiency at the nanoscale level. The mechanism behind the hot electron injection or charge transfer from metal nanoparticles to a semiconductor is not yet fully understood and many experimental and theore- tical studies are in progress. The present understanding is based on the hot electron or hole transfer, light trapping due to scattering at the interface, and plasmon resonance based energy transfer. Some of the difficulties in understanding this complex phenomenon include strong light interactions/ couplings with plasmons and the fact that the dynamics of hot carriers lifetime is in the range of a few femto to pico seconds. The study on the colloidal quantum dots coupled or grown on nanorods or nanoribbons has shown very good progress in a Academy of Scientific and Innovative Research (AcSIR), National Physical Laboratory, Council of Scientific and Industrial Research, Dr K. S Krishnan Road, New Delhi-110012, India. E-mail: husalesc@nplindia.org b National Physical Laboratory, Council of Scientific and Industrial Research, Dr K. S Krishnan Road, New Delhi-110012, India † Electronic supplementary information (ESI) available: UTRPPS system descrip- tion, 3D schematic of Au–CdS, voltage dependent photocurrent, power dependent photocurrent, FDTD simulation of Au–CdS film, transient absorption spectra of only Au film. See DOI: 10.1039/c6tc04243k Received 29th September 2016, Accepted 13th December 2016 DOI: 10.1039/c6tc04243k www.rsc.org/MaterialsC Journal of Materials Chemistry C PAPER Published on 13 December 2016. Downloaded by National Physical Laboratory (NPL) on 10/01/2017 07:02:16. View Article Online View Journal