A single-electron transistor (SET) was fabricated using a cadmium selenide (CdSe) nanocrystal. The nanocrystal, with a diameter of 5.5 nm, was attached to two gold leads using bifunctional linker molecules. The device was fabricated on a degenerately doped silicon wafer, which served as a gate to tune the charge state of the nanocrystal. The device was used to measure the energy required to add or remove electrons from the nanocrystal, demonstrating Coulomb blockade effects. The conductance of the device was measured as a function of gate voltage, showing Coulomb oscillations when the charge state of the nanocrystal changed by one electron. The differential conductance was mapped as a function of both gate voltage and voltage applied between the leads, revealing Coulomb gaps where no current flowed. The Coulomb gap size varied with gate voltage, and the maximum gap size corresponded to the addition energy for the next electron. The addition energies for successive electrons were determined to be between 15 and 60 meV. These results were consistent with the Coulomb blockade model, which predicts that the addition energy is the sum of the Coulomb interaction energy and the energy level spacing. The results suggest that the addition energy for individual holes on a CdSe nanocrystal is smaller than expected. The study provides a new type of spectroscopy for single nanocrystals, allowing the measurement of the energy required to add a single type of charge carrier. Future studies will investigate how the ground state and excited state properties of nanocrystals vary with size, shape, and composition. The work was supported by the U.S. Department of Energy and the Office of Naval Research.A single-electron transistor (SET) was fabricated using a cadmium selenide (CdSe) nanocrystal. The nanocrystal, with a diameter of 5.5 nm, was attached to two gold leads using bifunctional linker molecules. The device was fabricated on a degenerately doped silicon wafer, which served as a gate to tune the charge state of the nanocrystal. The device was used to measure the energy required to add or remove electrons from the nanocrystal, demonstrating Coulomb blockade effects. The conductance of the device was measured as a function of gate voltage, showing Coulomb oscillations when the charge state of the nanocrystal changed by one electron. The differential conductance was mapped as a function of both gate voltage and voltage applied between the leads, revealing Coulomb gaps where no current flowed. The Coulomb gap size varied with gate voltage, and the maximum gap size corresponded to the addition energy for the next electron. The addition energies for successive electrons were determined to be between 15 and 60 meV. These results were consistent with the Coulomb blockade model, which predicts that the addition energy is the sum of the Coulomb interaction energy and the energy level spacing. The results suggest that the addition energy for individual holes on a CdSe nanocrystal is smaller than expected. The study provides a new type of spectroscopy for single nanocrystals, allowing the measurement of the energy required to add a single type of charge carrier. Future studies will investigate how the ground state and excited state properties of nanocrystals vary with size, shape, and composition. The work was supported by the U.S. Department of Energy and the Office of Naval Research.