Colloidal PbS nanosheets represent an important infrared 2D nanomaterial. They have tunable energy gaps and much higher charge carrier mobility than quantum-dot films, which makes them ideal for optoelectronic and electronic applications including photovoltaic devices and field-effect transistors.

The growth of PbS nanosheets is confirmed by electron microscopy and photoluminescence spectroscopy. Their thickness can be tuned by changing the reaction temperature during the synthesis. Recent research also demonstrates that the lateral size of the nanosheets can be systematically tuned between 20 nm to a few hundred nanometers. Core/shell PbS/CdS nanosheets can be synthesized using a cation exchange method. This method can protect the PbS core as well as further tuning the core thickness. Surface passivation of the nanosheets using organic molecules is also proven to be very effective to suppress the surface defects and improve the optical properties.

The energy gap of the PbS nanosheets can be tuned by changing their thickness. The thickness dependent energy gap is a unique feature of the exciton under one-dimensional confinement. In contrast to quantum dots, the confinement energy can be achieved in a typical nanosheet is smaller than a quantum dot. However, the maximum energy gap of the nanosheet can still reach 1 eV which is about more than twice of the energy gap of the bulk PbS.

The optical absorption of typical PbS nanosheets shows a step-like spectrum. The step edge is nearly coincident with the photoluminescence peak, indicating a negligible Stokes shift. The absorption spectrum of the nanosheets of 20 nm in lateral size shows an excitonic peak similar to quantum dots, which is possibly due to the additional lateral confinement.

The absolute photoluminescence quantum yield of the PbS nanosheets can be measured accurately by using an integrating-sphere technique. Well-passivated PbS nanosheets typically show 20% to 30% photoluminescence quantum yield. The maximum photoluminescence quantum yield has reached 60%, which is about twice of the quantum yield from PbS quantum dots of the same energy gap.

The time-resolved photoluminescence of the PbS nanosheets shows a distinct fast decay followed by a slow decay. The fast decay accounts for more than 90% of the total luminescence intensity. The exciton radiative lifetime derived from the time-resolved photoluminescence and the quantum yield is much shorter than the PbS quantum dots. It is an indication of giant oscillation strength transition which is a consequence of large exciton coherence volume in a nanosheet.