The effect of hole localization on photocatalytic activity of Pt-tipped semiconductor nanocrystals is investigated. By tuning the energy balance at the semiconductor-ligand interface, we demonstrate that hydrogen production on Pt sites is efficient only when electron-donating molecules are used for stabilizing semiconductor surfaces. These surfactants play an important role in enabling an efficient and stable reduction of water by heterostructured nanocrystals as they fill vacancies in the valence band of the semiconductor domain, preventing its degradation. In particular, we show that the energy of oxidizing holes can be efficiently transferred to a ligand moiety, leaving the semiconductor domain intact. This allows reusing the inorganic portion of the "degraded" nanocrystal-ligand system simply by recharging these nanoparticles with fresh ligands.

Ultrafast transient absorption spectroscopy was used to investigate the nature of photoinduced charge transfer processes taking place in ZnSe/CdS/Pt colloidal heteronanocrystals. These nanoparticles consist of a dot-in-a-rod semiconductor domain (ZnSe/CdS) coupled to a Pt tip. Together the three components are designed to dissociate an electron-hole pair by pinning the hole in the ZnSe domain while allowing the electron to transfer into the Pt tip. Separated charges can then induce a catalytic reaction, such as the light-driven hydrogen production. Present measurements demonstrate that the internal electron-hole separation is fast and results in the localization of both charges in nonadjacent parts of the nanoparticle. In particular, we show that photoinduced holes become confined within the ZnSe domain in less than 2 ps, while electrons take approximately 15 ps to transition into a Pt tip. More importantly, we demonstrate that the presence of the ZnSe dot within the CdS nanorods plays a key role both in enabling photoinduced separation of charges and in suppressing their backward recombination. The implications of the observed exciton dynamics to photocatalytic function of ZnSe/CdS/Pt heteronanocrystals are discussed.

Two-dimensional semiconducting systems, such as quantum wells and transition metal dichalcogenides, are the foundations to investigate low dimensional light-matter interactions 1,2 . To date, the study of elementary photoexcitation, namely the exciton, in 2D semiconductors with intrinsic magnetic order remains a challenge due to the lack of suitable material platforms 3,4 . Here, we report an observation of excitons coupled to zigzag antiferromagnetic order in the layered antiferromagnetic insulator NiPS3 using both photoluminescence and optical reflection spectroscopy. The exciton exhibits a linewidth as narrow as ~350 µeV with near unity linear polarization in the photoluminescence spectrum. As the thicknesses of samples is reduced from five layers to bilayers, the photoluminescence intensity is drastically suppressed and eventually vanishes in monolayers, consistent with the calculated bandgap being highly indirect for both bilayer and monolayer 5 . Furthermore, we observed strong linear dichroism over a broad spectra range, which shares the same optical anisotropy axis, being locked to the zigzag direction, as the exciton photoluminescence. Both linear dichroism and the degree of linear polarization in the exciton photoluminescence decrease as the temperature increases and become negligible above the Néel temperature. These observations suggest both optical quantities are probes of the symmetry breaking magnetic order parameter. In addition, a sharp resonance in the linear dichroism spectrum is observed with an energy near the exciton photoluminescence. There exist over ten exciton-A1g phonon bound states on its high energy side, which likely result from the strong modulation of the ligand-to-metal charge transfer energy by strong electron-lattice interactions. Our work establishes NiPS3 as a new 2D platform for exploring magnetoexciton physics with strong correlations, as well as a building block for 2D heterostructures for engineering physical phenomena with time reversal symmetry breaking.

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